I shoot basketball games in large arenas and the lighting is going to be much brighter than those of HS gyms and recreation centers. I use a Lastolite Ezybalance Greycard.
http://www.lastolite.com/ezybalance.php
It makes a huge difference and just takes a second. I am about to go to an ExpoDisc very soon but the Lastolite has worked great for me. I shoot a Nikon D300 and a Nikon 70-200mm 2.8 for NBA games.
All I do is have someone hold the grey card for me, fill the frame with it and shoot it where the action will be. Then I select that image for my custom white balance setting in my camera and done! That simple. Good luck, I'm sure there are some great people here that can also give you their way but that's worked for me. Good luck.