I bring a cushion like this.
Also the Hilton features the finest tables made of some beat-up plywood with some plastic trim scraps, staples and a few splinters. So bring a mouse pad and wrist rest if your arms usually rub the edge of the table while gaming.
If you don't want to survive on pizza and burgers for 3 days, pack some fruit or granola bars or something.
The tournament finals on Friday will have free beer. It's all you can drink if you're friendly to the staff or round-robin between each bar.
During master pancake theater, the crowd will discreetly carry in maybe 300 gallons of hard liquor. It's usually great fun. Prepare as you see fit. Please don't scream like a banshee during the show though.