I have to second the suggestion of waiting to make your purchases in the dealer room. Not only do you have to lug all that stuff around all day, you're going to be more likely to give in to impulse buying. You've got plenty of time, so take a couple of looks at a book, see if you can get in on a demo for a new game.
I haven't been to GenCon Indy (hopefully this year), but in Milwaukee I found that taking just a short walk from the con made a bid difference in what was available for food. I found you could get a really good burger and fries in a nearby hotel restaurant for about the same as what you'd pay for much less inside the convention center. I'm not sure how the food is in Indy, but it would probably pay off to scout around a bit.
If you're going with friends, establish a time and place to meet regularly. You'll want to swap ideas on games, tips on stuff to check out, and schedule any other activities you plan to do together. Otherwise, you'll spend half your trip wandering around looking for each other.
Take an ample supply of cash. Not all vendors are plastic or check friendly. This is probably less true than in the past, but it will invariably be the person selling the thing you want the most.
You must have the basic adventurers survival kit. Pack light, but include the essentials. Backpack, dice, a couple pencils, and a notepad. Most other supplies you need will be provided for you at most games. Also include a bottle of your favorite pain reliever (Advil, Tylenol, etc.). Most gamers (or at least myself) spend as much time on their feet at GenCon as they do the rest of the year combined.
If you have time, make one last trip through the dealer's room on Sunday morning. You'd be surprised at the kinds of deals you can get from people who don't want to pack all that stuff back up.