How to score on Craigslist -- please give me advice  

Well, for one thing,
it's very common for people with no experience with the hunt to get the impression that Craigslist (or auctions or yard sales or whatever) are bulging with incredible finds just waiting to be snatched up for nothing.
This is not the case. There is a ton of junk out there. Lots of Sears colonial furniture, lots of IKEA, lots of just plain garbage.
What you don't hear about when someone says, "Look at this set of six Danish rosewood dining chairs I got for $50 on Craigslist!" is "I spent hundreds and hundreds of hours checking Craigslist for something good, scanning page after page after page after page of stuff that interests me less than a dust mote, before I came upon this!"
I got an awful lot of great stuff at auctions over the years and friends would inevitably get the impression that one could just walk into a local auction, find something great, bid on it and get it for under $10. But really for every incredible find, there were 20-50 (or more) hours spent waiting, hunting, being bored out of my mind, hot and sweaty in summer, freezing in winter, in the company of a lot of dishonest and unpleasant people, and getting way too many whiffs of someone else's coffee/cig breath and/or BO.
Anyway, short answer: keep at it, you'll find something.

Patience, dumb luck, and a plan.
CL is a black hole of warped and wasted time... if you let it suck you in. Believe me, I know. But I have gotten a couple of my favorite things at good prices off CL and here's how:
Be patient. It takes time to learn your market. Realize that you're gonna miss out on good deals and get over it. It'll only happen if it was meant to be.
Decide on three or four specific items you want and limit your searches to key words specific to those items.
Limit your time searching CL to under a half-hour and try to do it at or near the same time every day. (For me it was 6:45 - 7:00AM Mon.-Fri.)
Limit your geographical radius to a drive (round trip) you'd be willing to make on a moment's notice.
Have cash and be ready to pull the trigger. The good deals are often made in under an hour from the initial listing.
Don't buy junk or pay retail. Stick to your standards for condition and service. If they're asking a gallery price, you should get gallery quality and service in return.
Bon Chance!

Ditto what spanky and tk said.
Ditto what spanky and tk said.

Thanks for the reality check....
Yes, everything you all say makes perfect sense, and Spanky, you put it very colorfully and gave me a laugh to boot.
Of course just drooling over the look-what-I-scored threads gives a false impression. I hope one day to post a find of my own!

Two words
Magical Dumpster.

One thing I've noticed recent...
One thing I've noticed recently is dealers totally abusing the free listing of CL. In the Bay Area, some shameless dealers would literally put 100's of items on CL everyday using keywords that would cover every major designer and period. Not only that, they'd be renewed every single day. So when I browse on CL, I'd see the same crap multiple times a day. In fact, most of what they sell is crap. It's really frustrating because it makes it even harder to find anything meaningful.

I suppose it depends on your...
I suppose it depends on your local CL, as there are a lot of differences from one locale to the next, but my experience is that the real deals require hard work. Anyone can set up a push RSS feed and get an alert 30 seconds after someone posts an ad with certain keywords in it. Couple that with the ease with which the sellers can search eBay and get an idea for the value of their item, and it (in my opinion) seriously limits the usefulness of keyword searches.
If you really want the good stuff, and you want it cheap, you shouldn't bother clicking the ad titled "Vintage Herman Miller Eames lounge chair and ottoman in rosewood". The ad you're looking for is "Old brown chair". You can't search for that, you have to pore over *every* listing. And there are typically others doing the same (especially dealers), so you have to do it all the time. This is my hobby, not my profession, but on the days I'm "hunting", I'm searching every 20-30 minutes to look at everything that's been posted since my last browse. If I'm lucky, 6-8 hours of this will yield something decent (not spectacular). That may or may not be reasonable for you, but where I'm at (not even a major market), that's what it takes.
As for deciding which pieces to pursue, I'd follow your gut and go after anything that looks good to you. Also, talk to your local dealers and get an idea what they're willing to give (in trade) for the pieces you acquire but don't want to keep.

That's what the flag button's for, kin...
But then who the hell has time for that, eh?

Patience & persistence is key...
Patience & persistence is key on CL.

know your stuff and always...
know your stuff and always be nice and polite to the seller even if the item being offered for sale is flawed or you felt the drive was a waste of time. avoid being critical of the item for sale (to get a good deal) and never correct the seller if you think they are giving you incorrect information or you think the sellers family story about the provenance or history of the item is a load of crap. just keep nodding your head, be nonchalant because sometimes it is not what is posted that is lurking in the sellers property that you will end up taking home for a much better deal.
i ended up with a pair of grungy bruno matthsson tables that the seller thought looked so out of place with their spanking new stuff from pottery barn. they also thought their grandparents taste in furniture was weird/bizarre and looks like they all came from a cafeteria or bowling alley.

are you a DIYer?
It also depends on how much time you're willing to put into refurbishing a piece. Obviously this depends on if you're restoring a vintage teak credenza or an Eames shell chair, but I've picked some grungy looking stuff for really cheap that looked brand new with a little bit of elbow grease.

Sometimes (but rarely)
I will offer more than what they are asking if I think there is a good chance that someone else has seen what I have seen. I will put a higher dollar amount in the subject line of the email to get their attention. This has worked for me a few times. The best advice is to check CL and check it often. Always include a phone number in your email. And be ready to drop everything you are doing inorder to best meet the sellers schedule.

to echo some points above
#1 - most important - Know what you're looking for. You need to be able to pick out a gem by the photo(s). If a person lists a pair of Erik Buck bar stools as "Erik Buck Bar Stools" they obviously know what they have and will likely ask a price worthy of what they have. If you happen to search for "Mad Men" and see a pair of bar stools listed as "2 Funky Mad Men Bar Stools $100" and they just happen to be Erik Buck bar stools
#2 - Be nice and detailed in emails. In the chance where the poster likely will get numerous inquiries, they will probably respond to the nicest sounding email first. I usually lead with "Wow! That (insert item here) is amazing! I've been looking for something just like it. It would look perfect in my house!"
#3 - If something is super awesome and cheap - don't ask for a lower price. If you low-ball you will likely get overlooked by someone offering full price. Email or call them. tell them you'll pick it up that day and pay full price.
#4 - If you see something super awesome, but expensive, don't be afraid to mega low ball, but do so in a polite way. "Wow, your (insert item name here) is amazing! I wish I could fit it in my budget. I don't want to offend you, but if you find yourself needing to offload it, I could offer you $X for it. Best of luck! It really is an amazing piece."
#5 - think of any crazy way someone could describe an item. My go-to searches are Funky, eames, retro, vintage, mcm, mad men, jetsons, 60's, (see my mad men erik buck story above)
#6 - Act fast. If you see something awesome, don't sit around contemplating it. Pounce! You're not the only one out there looking.
#7 - Know your level of comfort with refinishing. If you're not willing to re-upholster a chair, don't buy the chair that needs it. It'll just sit there being ugly.
#8 - Use Searchtempest or similar website that allows you to search multiple craigslist cities at once. You might find something awesome in the next city over.
#9 - Have fun! it's like a treasure hunt. When you do find something awesome, it's a great feeling!

Thank you,
That's a real treasure trove of information! I am good at the polite and enthusiastic, and not one to super lowball... But so far have not found much of anything good. Oh, except a super-cheap huge round Thayer Coggin sectional that in no way would've fit into either my vehicle or my living room. I will soldier on, thanks again for the advice and encouragement.