Shipping marble and glass table tops  

These sorts of things are typically packed individually, with thick blocks of foam around the edges, snugly encased in thick foam lined cardboard
always carry upright

Yes--always keep them on edge, never flat! This is true of anything breakable and flat, even dinner plates.
Also, use a rigid foam like styrofoam, not the stuff used in sofa cushions. That will get compressed flat in no time at all. Closed cell foam might be ok but make sure it's thick enough.
I would build a crate, too. It's not that big a deal. Home Depot and Lowe's and places like that will cut plywood and 2x stock to your specs if you don't have a power tools at home, then all you need to do is put it together with a drill and screwdriver bit.

Thanks guys, that definitely helps. I look into the foam blocks. Also, didn't know Home Depot and Lowe's offer cutting services, that'll make this much easier to deal with.

Oh, also, the big box stores will tell you that they will not guarantee the accuracy of their cuts to an absolutely precise degree but it's not to the point where it will make a difference for your needs.
I bet their are a ton of youtube videos about making shipping crates. That would be my next stop if I was doing this!

If you're near a larger city, it might be worth your while to get a quote from fine art shippers or other crating services for a one-way slat crate or two for your table tops. Art museums and better galleries are good for references.
I spent a few years designing crating systems of all kinds for what was, at the time, the largest fine art transit service in the US and we made many a crate for stone and glass furniture tops. The dynamics involved can get complicated, especially if you're not personally handling your move. Most of the big, national household movers offer specialized packing services, too. Sometimes, it's just worth it to defer to the pros.
Or, if you're the handy type, have basic tools and skills, and decide to tackle it yourself, I can walk you through it. It's not that difficult. You'd be looking at about 4 hours and maybe a couple hundred bucks plus some running around to source materials.

DAUser,
When I shipped some heavy and thick flat glass I began the project with flexible foam pipe insulation sold in "big box" hardware stores around the edges (this make handling easier). The addition of rigid foam as well as wood crating and corrugate were required prior to glass shipment. It arrived in fine condition.
Your local commercial glass shop should be able to show you their glass shipping crates (for both incoming and outgoing product). Copy these. Common sense should prevail.

I've moved twice with a Noguchi table. Both moves were about 150 miles (not quite cross country). I didn't trust putting the glass in a moving truck. I wrapped it it blankets and put it in my trunk. I want a Saarinen marble dining table but I'm sticking with my ikea knock off until I'm sure I won't move again.

DallasGraduate,
Time and money spent on correct packaging will allow anything to be moved anywhere at anytime in a safe and secure fashion.
So many good examples and suggestions of correct moving procedures have been discussed that ignorance of the subject is nearly impossible. Failure to recognize and accept the facts is no reason to own an " ikea knock off".
It is a poor excuse. Better you than me.

Niceguy, I know an older gentleman who inherited a painting by Thomas Eakins that he liked so much that, for many years, he would take it along to and fro his summer residence by placing it flat and unwrapped atop all the other stuff in the back of his crappy old station wagon for the several hundred mile drive. It was, after all, just a painting not so different than any other.
Then, he lent the picture to a museum for an exhibition which required an insurance appraisal. As you might guess, that put an end to trips in the family car.
That said, nothing beats careful handling. I, myself, have moved the top of my IN50 flat in the back of my station wagons a couple of times with nary a second thought and without incident. Handing it off to heaven-only-knows, however, would be another matter altogether.

My scenario is different. I've moved 6 times in the last six years. A nomadic lifestyle encourages me to minimize acquisition of items that need much more than a moving blanket and things that are heavy.

tktoo,
You and I both know things done right may not be as simplistic as they seem. Nothing, and I mean nothing, will alleviate an individuals (older gentleman story) unreasonable and sometimes imagined fear of material loss.
Moving precious objects from one location to another involves a risk and is tedious at best. All this aside, and with negative anomalies included I see no reason (only excuses) for point A to point B transit.
Again DallasGraduate, the frequency of your moves is an excuse not a valid reason for living with inferior products (throw away mentality). Better you than me.

Your screen name is niceguy but you don't seem very nice. You come off as a rude snob. Sometime you have to "suffer" use of inferior products until you are in a position to take care of more desirable items. How many times have you moved a household of items? Specialty movers are expensive and unless it has sentimental value a cost benefit analysis may suggest is better to sell and buy a replacement when you move. I acquired my dining room chairs because someone was making a move from Texas to New York. Keep in mind a broken marble top has little value even if it comes with a knoll sticker.

throw away isn't throw away if you don't throw it away

DallasGraduate,
Owning product incapable of passing a "cost benefit analysis" is nonsense. Employment by corporations that expect and respect actions versus excuses and in turn will pay for "specialty movers" of valued transferred employees is factual not snobbery.
The lack of shoe polish or the difficulty of application is no reason to go barefoot. I do not suffer fools and my screen name is a reflection of others reality.
I am grateful Ray Bradbury did not graduate.