Dried fruit and granola bars will keep your hunger satiated, along with plenty of water. Then I just pack a sandwich. Something you might want to do is unwrap the granola bars and put them in a plastic baggy (no sound while on the stand). A roll of toilet paper is a must.

Now, on how to do it - my method is different from somebody like my dad. I'm the most patient stand-sitter you'll ever see, so I can just get into a stand and stay there, period. What my dad does, though, since he's a little more antsy, is have three or four stands picked out with definite routes between them. Get in stand A, sit it from daybreak to.. say 9 or 10, then get down, stretch your legs, and walk to stand B. Sit that till mid-afternoon, then move to stand C for the evening. Having a walk to look forward to helps, rather than thinking 'man, it's 11:30, and I've got like 6 more hours to sit.'

Another way to do it is to bring, say, 5 or 6 things to do or look at... like a particular snack, a magazine article, the sports section, or going to the cell phone in the truck to say hi to somebody. Then you have a schedule, which helps you look forward to the next part of your day.

Good luck!