Before your toke session, prepare something healthy for yourself (i'd suggest apples & peanutbutter), and try to stick with just what you've prepared.

Easier said than done, I'm no wiz at bypassing the munchies, so if it's a problem, all you can do is deal with them appropriately.

There was about a year when I was very into physical fitness (before it smoking started catching up to my lungs) that I was able to completely ignore munchies for about that whole year. But I mean, who doesn't like to indulge?