As a healthcare provider (EMT Advanced and 4 weeks away from RN), I would second the hydration bladder idea. If you're sweating at all, you should be putting away between a half and a full litre per hour. Even if you're taking it easy, in a four hour game you should put away at least a litre. Most Americans are chronically dehydrated, anyhow.
I would also heartily recommend Blackhawk with two caveats:
1. The colour they promote as "Coyote Brown" or "Khaki" has a lot more in common with OD Green than any brown or tan I've ever seen.
2. It is not made in the US; ironically, most of their gear is made in 'Nam.
For one of my day jobs, I wear a modular vest by Spec Ops Brand that I'm EXTREMELY satisfied with; I'd consider picking up another for airsoft if it weren't so fracking expensive.
Good prices can be gotten on Blackhawk, Surefire and Spec Ops from
LA Police Gear. I'm not connected with them in any way other than being an satisfied customer ever since I used them to replace some gear I lost responding to Katrina.
AEX also carries Blackhawk at decent prices, plus has the advantage of being able to fit-test in the showroom.
Peace,
Hans