Oh, oh, me, me!
It all depends on
a) how much you are willing to spend.
b) what king of riding you are doing.
I've used a few different methods, all using a digital camcorder.
None of them have been for track use, but for touring footage.
1)
Put a proper tripod mount on camera base, and sit on top of tank filler cap (with a bit of padding).
Use loadsa industrial strength gaffer tape to fix to tank.
If your camera has a decent stabiliser, turn it on.
Turn on any wind filter, or fit external mike out of wind.
Screen will shield camera from midges, etc.
Clean your screen, and try not to have many scratches.
2)
Buy one of those small tripods with suction feet to fix camera to tank.
This is sometimes allowed on the track (witness Big Nige's footage)
With these, you can adjust the height and angle of the camera
I didn't feel confident with just three suction cups holding things on.
NOTE: With the above mountings, the camera can get in the way of your chest, so it really is best just for off-track use.
3)
Put your camera on it's full-size tripod, but with the legs still folded.
Tilt the tripod head 90 degrees
Strap the tripod _firmly_ across the pilion seat, so the camera now points forward.
All mounts must be _tight_, and the footage will be shakier than above.
4) Hand held on left hand, with back of hand to camera body.
very shaky, and probably illegal.
5)I've not tried this personally, but hope to this year.
Buy a pencil camera kit to fix to any part of the bike you want, putting the camera itself under the seat, but preferably in a backpack to isolate vibrations.
Best to have a remote control in this case, although the wired LANC type are notoriously expensive, and usually have short cables (they are meant for tripod pan handles)
The camera send it's output to the analogue input of the camcorder.
For example footage, have a looksee at...
Scottydog's websiteMost footage is stable, except the hand held stuff crossing the Skye Bridge.