Thrift shop finds?  

persistance
For every great find I've gotten at a thrift store, there have been a LOT of visits when I found nothing at all.
It also depends on the area. Some parts of the country are better for bottom feeding than other parts. Some thrift stores are better than others. Here in the Baltimore area you can find stuff but damn, you have to be QUICK. Almost every time I am in Goodwill, I see dealers I know personally. If I see ones I know, then there are probably other dealers I don't know. That's a lot of dealers.
I offset the few finds I've gotten by donating lots of clothing and household goods. I just got home from an estate auction where I scavenged a boxful of perfectly good, useful stuff to take to Goodwill.

Thrifting for fun
First, I encourage you to continue feeling stupid driving your hummer regardless of where you shop. : ) Drive it to a good bike shop.
Second, the joy of thrifting for me is finding interesting vintage objects. I once found an amazing nixie tube calculator at a thrift store, for example, beautiful in design, yet completely out of production and out of fashion. The fun was finding it, exploring it, seeing every authentic curve and vintage line.
Honestly, I never expect to find something of great market valuable. But how about an incomplete set of mid-century restaurant ware? Gorgeous danish modern barkcloth curtains? Ah, and that Donna Summer album that includes a fold out poster of Donna over the words "I Love You". It's an adventure, Riki. For me, that's all. But it's therapy and relaxing.

Thrifting for value
The other importance of thrift stores is that people with little money can buy quality second-hand things for low prices. They don't have to ask for charity, and they don't have to buy particle board shite from wal-mart. That's if the thrift stores are being run well. Unfortunately, it's not uncommon today to find a local thrift store actually selling its most valuable goods on ebay, or they consolidate those goods in stores located in more upscale or trendy parts of the city where they know college hipsters and SUV owners will pay top $. The money, they say, goes to feed and clothe the poor through charity. Maybe. But unfortunately it also robs a certain independence from the poor who might have loved the Eames chair, despite not knowing who Eames was.

Where do you live?
The pickings are inevitably better in metropolitan areas-- where there's better stuff, there're better cast-offs.
The trick to thrifting is patience and persistence-- I probably average one purchase per ten visits (sometimes I find only a few pieces of Norwegian stainless flatware... other times, a good piece of Danish furniture). Frequent visits translate to greater success-- the gems don't sit around gathering dust, if you're not there to find them, someone else will be.
And, don't feel as though you're poaching on the territory of the poor, because: A) thrift shops exist for the sole purpose of raising funds for their organizations-- NOT to provide cheap goods for the poor. Charitable thrifts welcome EVERYONE'S patronage. B) Lower class people wouldn't be caught DEAD with fifty-year-old pieces of furniture in their homes, they scope out the puffy 1990's leather sofas, and reproduction Tiffany lamps. No conflict.

I would definitely feel stupid driving a Hummer
into a thrift shop parking lot. And if you feel you should be handing out dollar bills to the folks who look like they need it, then by all means, do so.
Regionality will certainly play a role in what you might find. Over time, you learn which stores in which areas are good to go to. For example, a nearby town that's known for it's antique shops has really lousy thrift stores. My guess is because all the dealers clean them out very regularly. Or, the franchises know not to send their "good stuff" to those areas for that reason - I don't know the real answer.
As for thrift stores being loaded with "junk" -- the thing is that's how all thrift stores appear, until you spend time learning how to see what you're looking for. It's not obvious. It takes lots of time to really look closely at things, because the way they are "displayed" (ie, heaped in piles) tends to be like a sort of camouflage - it's the way that hidden treasures protect themselves.
I enjoy much more the time I spend in thrift stores vs the time I spend in an "antique" store. I like the thrill of discovery, the hunt - in antique stores, it's all just laid out and tagged and described, just no fun at all.
As for how stuff ends up there - remember that there are plenty of people who see the shiny new "retro" stuff at the stores and can't wait to trade in their old furniture that has scuffs on it for the brand new versions. They don't know the difference, except the new stuff is, well, *new* and seems to look better to them. Good for me, that they do this. They replace actual good designer stuff with China-imports.

[Not to
ride our new friend too much, but there's a nice take on "handing out dollar bills" in favorite columnist Jon Carroll's annual promotion of the Untied Way -- a riff on the United Way Christmas Appeal. He says "go to the ATM and take out as many 20-dollar bills as you are comfortable giving away, and hand them to those on the street who say (or don't say) that they need some help. Don't expect them to spend the money the way you would; don't have any expectation at all. But know that at least all of what you give is going directly to (some) needy people. . ."]
A Hummer ? Nice toy -- how much will it hold ? Or isn't that relevant ? (The H2 is pretty snazzy -- I note that several others are copying the look, to one extent or another; Jeep now has a model that seems to echo its lines. When Rover does that I'll know the circle has been completed.)

i never get any luck from thr...
i never get any luck from thrift shop here, down under. i used to go to this shop everyday, but one day a friend of mine who goes there one in 3 weeks, got nice designer buffet, persistence? just about luck ..

Persevere
The commonality in all of this advice appears to be perseverence and so I shall keep on keepin' on. To address your other points, I am in South Carolina where people from other places go to retire, not the other way around, and therefore you don't have tons of folks cleaning out their homes when they retire and downsize. There is also alot of poverty here and a good portion of the population just doesn't have anything great to donate in the first place. I'll bet the thrift stores in Atlanta have better stuff, I'll have to broaden my search area.
On my choice of vehicle, ahem, I was in a horrible car accident in my 20's and I have a phobia of small cars. If I could drive a Sherman tank, I would. My H2 is the next best thing. This my second one, before that I drove Land Rovers but they didn't hold enough "stuff". The H2 could hold the Land Rover! It's great for going to auctions close to home. Further excursions, I drive my pickup. Remember, gas in SC is cheap, cheap compared to y'alls so we don't have that crazy expense here. (Yesterday, $3.51).
A bike? Moi? I might sweat and I don't do sweat.:)
Dashes, don't rule out antique stores. Go to big antique "Malls" that have alot of different dealers under one roof. I got a conical planter in a hairpin leg stand that somebody had painted barn red trying to make it look "country" for 20 dollars. I painted the stand black and the pot white and it looks great!

Love Thrift Shops....
I found my glassware, some more of the ironware service I have, a Paul McCobb dining table (beaten up, but works well as a clothes folding table down near the laundry), a real Saarinen tulip oval side table, and lots of other cool stuff.
As far as a Hummer, well, you mentioned you had one, and I wouldn't touch one with a 10' pole.

auctions
You mentioned going to auctions. I've done way, WAY better at auctions than at thrift shops. It takes a lot more time but it's worth it for me. I go to one sale every week and other less regular ones and I've made a lot of friends among the regulars over the years. This helps when there's a lot of waiting around. We sit and talk about all kinds of stuff.
Last week I got a glass vase by Bengte Orup for Johansfors for $9. Woohoo!! Last night I got a Selig lounge chair (the Eames knockoff) for two bucks, but it needs extensive work. Got an old Kevi office chair for two bucks, too, but it's missing one little knob on the back, which I didn't notice until after I'd gotten it.

Location Location Location
thats the trick to consistent good finds in thrift stores. If the area wasn't ever a host to MCM you probably wont find MCM in the stores. case in point: I drove through southern NH one day and stopped at every thrift,junk,consignment shop I could find. Not a single damn piece.
However, the junk shop near my house in Cambridge consistently turns up Eames pieces, Lane coffee tables, odd little benches, all kinds of neat stuff. And of course, persistence is key. The junk shop also turns up lots of 'shabby chic' and lots of paintings of Jesus.

What I remember from when I...
What I remember from when I used to scout these types of shops more frequently is that most of them had one shop area and then one much larger storage/warehouse area somewhere in the back of the store.
The staff would be bringing out items from the storage only as people bought the stuff that was already out on display. There never seemed to be any thought to what they would bring out, at least it just seemed completely random.
Every now and then, but very irregularly, something good would be brought out when I happened to be there. And almost every time someone else would snatch it right in front of me. I guess I was never audacious enough. But it always seemed as if someone "in the know" was there at all times, circling the prey so to speak. And I guess it is safest to assume that is the case.
So my advice would be to go there just before they open, run inside, pick up anything that looks remotely interesting and hold on to it as if your life depended on it, until you are 100% sure you are not going to buy it.
Actually, a friend of mine who visits flea markets every week tells me this is what she and the other "pros" do for real. Crazy, I say. But as WHC noted, low success rates should be expected. Anything else is just a bonus.

i have found very few thi...
i have found very few things at thrift shops and i to0 feel funny going in some of them,
I have done well at the flee markets and estate sales, and ebay,
For some reason, i have no idea why the great stuff does not end up at thrift shops,
I have only found the off brand stuff, and lots of little stuff, ceramics, and junk,
The fun of this deal is the hunt, hunting for cool stuff as i call it, you never know where you will find the great pieces.
I love the modern shops in Dallas, Kansas City,New york City,and Palm Springs found great clocks and furniture at those stores,

Good thrift Karma
I have a good thrift story.I was at a highly popular and therefore crowded thrift store about a year ago.The workers were constantly bringing stuff out from the back room,as people swarmed about.Like a shark tank at feeding time.I was standing near the front counter when the worker wheeled a bin out and placed a bunch of original 70's sealed in plastic Star Wars toys out on the counter.This big,fat "Slobba the Hut" kind of guy just about elbowed me out of the way as he grabbed up all 6 packages right in front of me.I was doing a slow burn at this point and went toward the coat racks at the back of the store to see if anything of interest was there.I noticed beneath the coats,all sorts of cast off luggage,from cheap plastic to vintage Samsonite.Just as I was bending down to look,I heard."Excuse me,"as one of the workers wheeled his canvas bin by me.He pulled out a sizeable trunk,and it had markings all over its weathered,tan leather trimmed exterior.I pounced on it immediately.Upon closer inspection,I nearly blacked out,as I realized it was an original Louis Vuitton travel trunk from the 1930's!!!Price? -$14.99!!!-Woohooo !!!!.I was shaking as I paid for it,thinking they were going to tell me it was mis-marked.(It would have been a bargain even if it were $149.99).Keep your plastic space toys,fat boy!!!