Wow... I feel like the only girl here who likes high heels. *runs and hides*
Seriously, in the winter I practically live in high-heeled boots, and I think they're really comfortable. Pumps and sandals are more of a crapshoot, comfort-wise, and I refuse to wear a pair of high heels that hurt my feet. I am not a living-in-stilettos fashionista by any stretch of the imagination, but in my experience I've found a general rule of thumb: you've got to shell out a decent amount of money for comfortable heels, and if they're any higher than 3 inches, they'll hurt. 2 1/2 inches is usually a pretty good combination of "stylish but not pain-inducing" if you've got the right pair. Not always -- once I had a great pair of high-heeled sandals that I could walk around in all day that I bought at Payless for $17, but I've never had luck like that since. You don't have to spend hundreds at Manolo Blahnik or anything *shudder*, but something like Steve Madden or Kenneth Cole is usually a good bet, especially if you can find something on sale. I'm not much of a shoe fiend anyway; investing in one good pair of black pumps will hold me for a few years.
Bonogirl: You started off with 4" heels?!
Wow. I'd like to think I'm comfortable in heels, but I can't handle them that high. I feel like I'm going to tip over and take ridiculous itty-bitty steps in them. If the fashion show will let you walk in a pair that's a little shorter, by all means go that route. But still practice in your 4" ones, because then the shorter pair will be a piece of cake!
But I agree with everyone else here; buy yourself a cheap shorter pair to practice in first and then work your way up. Also, make sure that you're practicing walking on hard surfaces, not carpet (that's also something anyone should do when trying on a pair in a store, too. Shoes might feel really comfortable on carpet but will destroy your feet once you hit tile/concrete/hardwood flooring.)
Oh, and remember: shoulders back, hips forward, and walk like a diva. Practice an "Oh yeah, I'm hot" face in the mirror, too.