most teams won't let you use flash if you're close to the field. Forget the flash.
Let's talk lenses, focusing, exposure, white balance, and RAW/JPG.
LENSES -- you need a fast aperature (f2.8 for zooms, faster if you can use primes) and a lens with a fast focusing mechanism (USM for canon, HSM for sigma) or a really quick hand at manual focus. A 70-200 f2.8 (not the f4) from either canon or sigma works pretty well. If you can do without the zoom, the 50f1.4, 85f1.8, 100f2, and 135f2 should be excellent. The 50f1.8 focusses to slowly to be useable.
FOCUS -- put the camera in AI SERVO mode. PIck the center focus point only (more accurate and faster). PRACTICE.
EXPOSURE: Set ISO1600 and get noise reduction software (neat image or noise ninja). If the lighting is fairly constant, use manual exposure. Leave Custom Function 4 set to zero, so all you have to do is half-press the shutter button to activate focus. If the lighting is variable, set Custom Function 4 to 3, and use the * button to focus. For sports like ice hockey make sure you're exposing the players correctly, forget about the bright glary ice. Select the wide open aperature (f2.8 if you're using the zoom lens), try a bunch of shutter speeds before the game, look at the histogram and pick your shutter speed. Dial in as M mode.
WHITE BALANCE: Bring a white card (piece of paper, etc...) or for ice hockey just use the ice. Don't use jerseys, t-shirts etc -- they're not really white. Purists use expensive gray cards. Whatever, the object just has to be color neutral. Take an out-of-focus photo of the neutral object, nearly filling the frame. If the object is white, make sure your exposure for this shot makes it look gray (eg, histogram should show a large spike near the middle). Now set the WB (white balance) to custom (setting uses buttons & dials on back of camera) and go into the menu to "custom white balance" to select your neutral object as the white balance guide. indoor swim meets are the worst.
RAW or JPG -- you have much better control with RAW, and can fix exposure and WB errors better, but the downsides are two: A) your CF card fills faster and B) in shooting rapid sequences, your camera buffer fills faster. I use JPG, but it's up to you. Along these lines, get a fast enough CF memory card. I use scandisk ultra II.
Some more hints:
- scout your best shooting locations and angles in advance.
- for kid's teams & school events, make sure you get each player
- start the game taking safe easy shots and get daring later
(easy shots in basketball are free throws, times when the players stop, passes, etc harder shots are jumps, dunks, and drives)
- watch for moments in play where the players motion is sideways rather than towards or away from you - focus is easier then.
- watch for your own safety. ice hockey from the penalty box, wear a helmet. basketball from the sidelines, watch for players falling on you. etc....