I have been to gencon six times now and have, barring getting sick a few times, enjoyed every one of them. These are the things I try to keep in mind when preparing:
1. Pick one or two days to have events, no more, and make them Thursday and/or Friday. Take events that intrigue you, regardless of system, and understand when you do that you're likely to miss about 30% of them from getting distracted by other things and that's ok
. Try not to schedule really late or really early 'cause that makes missing much more likely.
2. Find out where your friends are staying and try to get a nearby hotel. It's quite likely the things you'll most remember will come in the hotelrooms of those you party with each night so it's useful to not have to stumble too far when your drunk, exhausted, foot-sore, or any combination thereof.
3. Pack snacks. Seriously. A giant pretzel in the hall is $3.50 and sodas are $3.00, just like at ball games. There's a CVS in walking distance but the hours are horrible and the selection poor. When you are up drinking until 4AM, you'll be glad you remembered. Remembering band-aids for blisters from too much walking is also usually a good idea.
4. If you costume, leave yourself a 'normal' day if you also like to take pictures. Taking pictures and being in them is pretty much mutually exclusive. Alternatively, make arrangements with a buddy to follow you around with a camera and do it for you. I neglected this one this year and am left with NO pictures of my own of my getup and I'm rather bummed about that. Learn from my failings!