There is a lot of good information out there, but you could take a breeze through this video from a guy that like IDPA.
http://www.youtube.com/watch?v=If_oNKQSJrM&list=TLl9IeLXiGDr8huaxKCkqd2qKP70B-Y_6B
But apart from that the things you'll want are:
- baseball hat or some hat to keep the sun out and brass from getting down your glasses
- comfortable sneakers or shoes, you stand a lot most the day
- some sunblock - if you need that type of stuff
- bring water or something to keep you hydrated during the day
- snacks (though Harvard IDPA matches usually have a guy selling burgers and stuff)
- a small towel in the summer for your hands and sweat
- depending where you shoot, some bug spray, some clubs are nearer to swamp lands
- some people bring collapsable seats - but make sure you fit in them with your gear on
Most the shooting stuff was covered and readlily available, but these are some of the other things that are good to keep in mind. Remember, even slow shooting, you spend about 5-10 minutes shooting, the rest of the 5-6 hours is spent standing, taping targets and talking with some really cool guys and gals - some who are more sarcastic than others as you see.