I was nervous about stopping when I got my new bike this year because the top tube is so much higher than on my old hybrid. I was sure that I was going to come to a quick stop, drop forward from the seat, and do some serious damage! Fairly quickly, though, I taught myself this trick: When I need to stop, whether it's a quick'n'dirty or a gradual slow down, I decide which foot I want to land on, then as I put on the brakes I lean the bike in that direction, bringing that side of the bike closer to the ground so I can reach more easily with my foot. It's not a big lean, just a few degrees, but it's enough to whip my foot out of my PowerGrip (or toe cage, or clipless pedal, or off of a regular pedal) and get it on the ground. Then I pull my butt forward off the seat, and my foot and the two wheels end up forming a tripod under me. Does that make sense?

Now, if I were really cool, I'd learn how to do a cyclocross dismount (similar to what fatbottomedgurl and Tuckervill described). A good 'cross rider can swing one leg over behind the saddle, get both feet on the ground, then begin running while lifting the bike up onto one shoulder in order to jump over obstacles... And maybe, after I master that move, pigs will fly