Avoid 1080 unless you have a serious card in your pc that can deal with it. Vegas Platinum HD manages fine with 720 and thats all most people need. Free trials are available.
+1 for mounting upside down. Unless you want people to admire your toptube and knees instead of your riding that is...
Check lens regularly for mud/water. Annoying to get home and fine most of the footage is a big smudge.
Get a spare battery for use on longer rides.
Film other riders in front or behind and not just an empty trail. This means riding tight together(tighter than you would think) to keep them close in shot. Try to get alongside when the trail widens for a great tracking shot (if using helmet mount). This means riding with someone you trust and developing a whole new skill set. 
Get the camera off and film stuff in addition to usual rider's eye view. I put mine up in tree branches, under jumps or give to a mate to film of course!
Shoot B-Roll footage i.e. non biking stuff such as scenery, animals, tea stop etc. Helps break up the film.
If you post online don't expect the internet to go crazy over your footage unless you are doing big stuff. Your footage may look super rad to you but not so to others. You really appreciate how big some guys go on youtube when you watch your own footage of a 6ft drop and it looks tame.
On the same point, edit drastically. Youtube stats show when people get bored and its very easily!
And lastly, for best results remember to take your SD card out of the PC before you go riding!
Why? Mine doesn't need it. It was a pretty poky machine when it was new, to be fair, but it's years old now and still crunches 1080 files without too much stress.
