My friends and I play on a square mile of land, that has forest, structures, grasslands and creeks.
We don't play with referees, we use the honor system. (25 people, all know each other well, so no trust issues.)
We use full camo, full gear, and play like its for real, and those tactics carry over except for range.
Camo is the hardest thing to get right in different enviroments. Practicing like this keeps you on top of it, and lets you constantly check at how well you can blend in.
It is also useful for house clearing. CO2 tanks, and hoppers are about as clumsy as backpacks and extra magazine pouches, so it makes you pay a little more attention to noise discipline as well as picking your routes wisely with as copious cover as you can muster.
Also, you can modify your paintball gun to use your real type of grips, and add-on your rifles sights, because sight alignment practice, and aquire-fire drills can't be often enough.