When in doubt, gas it (especially in the dirt).
Ride on the wheel tracks, not between them.
Keep at least a 2 seconds distance between you and the vehicle in front of you.
In an emergency brake situation, take a quick peek in your mirrors. There's a good probability that the guy behind you didn't see you brake.
Always scan the road and look for the odd vehicle.
Keep your head cool. There's just no contest between a 200kg bike and an 1.5 ton car.
Drivers aren't evil (or, at least, 99% of them), they just don't see you. Make yourself visible. Add lights, horns, wear hi-viz. If you suspect one is going to cut you off at an intersection, swerve in front of him or stand on the pegs.
Stay out of blind spots. If you don't see the driver's eyes in his mirror, he won't see you.
As someone noted before, ride dirt. It'll teach you a lot faster and a lot less painful the limits of traction.
Take a track day, even if you've only got a 125.
Never push through a blind corner.
'92 Suzuki DR 350
'00 Ducati ST4