1) Keep the Camera Steady: For land shots I use a tripod and for tube/tow/chase shots I use both hands about 90% of the time.
2) Smooth Pans: Keep your pans smooth as possible. You have to learn to anticipate the riders movement and don't jerk the camera around.
3) Framing: This varies depending on the shot be it land, slider, tube, chase, or tow boat. Be creative and experiment. As long as you don't miss the trick then you should have something you can work with...
4) Good Lighting: If the light is in the rider's eyes then you're driving in the right direction. There's nothing worse than filming into the sun as your rider will be just a dark figure.
5) Misc: a) Bring extra batteries. b) Bring extra tapes c) Only use a lens cloth when cleaning your lens and always make sure you lens is clean and free from lint...
6) Start/Stop: What to film? Depending on the level of riding you're filming you can burn through a LOT of tape if you record every rider getting up and straight through their entire run. Use discretion.... Unless it's the person's first time then I wouldn't even bother taping them getting up. Also most beginners take more time between jumps so don't start recording until the rider is making their outside cut and then stop about 2 seconds after they land their jump. It makes it easier to watch and easier to edit.
7) Practice: After a day of filming review your work on a TV set. This will allow you to critique your own work while it's still fresh in your mind and you can correct your mistake for the next time you're out filming.
8) Insurance: Make sure you contact your home owners/renters insurance policy carrier and tell them you want to schedule your camera equipment separately. This will cover accidental loss like dropping it in the water. I pay $16/yr per $1000 value of equipment. (e.g. $3000 is $48/yr) Otherwise your equipment is only covered if it is stolen....