Lets start with pre-con preparation. I go grocery shopping for caffeinated cola, candy, and granola bars. The caffeine and the sugar with keep you awake, while the granola will keep you from passing out. Some people also buy beef jerky or power bars. Coffee's good to and if you have the money to buy it everyday twice or maybe three times a day do it. But I know I want all my money to go to shopping. It's bad enough that you have to pay for the hotel room, you don't want to wast your money. If you want to you can buy other snack food that can be kept in your room and drink, like juice or bottled water. I don't drink during conventions so I don't get tired, but it your going to drink why not bring your own.
Now I want to talk about food. During the con you need to snack to keep your energy up. Vending machine's are expensive, that's why you bring your own food. Also don't skip meals. I know you don't want to spend your money on food which is why you eat cheap. In most con handbooks there is a map of the surrounding area and a list of food vendors. Find the cheep places and just go there all weekend. I don't care of your sick of burgers and fries it's cheap. Also and this is very important KEEP HYDRATED. That means drinking water, soda does NOT hydrate you. There will be water everywhere. When you see water drink it even if you don't think your thirsty. You'll feel better for it.
Next sleep, I know there are people who come to con's with out rooms and try to stay up all night and sleep in Anime viewing rooms when they can. This works at Sakura-con since the convention is 24. People sleep is important. Even if you only get a couple hours, thats still sleep. When I'm at Sakura-con I'll go back to my room for a few hours just to lay down when there's nothing I want to do. If the time for my nap is short I won't even change out of my costume completely. I usually end up wearing costumes that have many layers. I shed the top layers so I'm comfortable and then sleep in the rest. It cuts off time when I'm getting back into costume to go back to the con.
So there are my rules to live by. Eat when you can, drink fluids, keep sugar and caffeine running through you at all times and sleep when you can. if your lucky you won't get sick until after the convention. I would recommend taking the day after the con off and sleeping. I hope this is useful to someone. Good luck at your con!