Basically you need to pull up on the front and at then pull up the rear asap right after the front. Its almost a rhythm that you need to get used to. Also practice makes perfectHow do you bunny hop on a bike? I keep trying but can only get up about about 2 inches max.
the rule is "down equals up". if you load down on the bike just before you launch, you'll find yourself airborn. i push down just before the front tire touches a log or other obstacle and usually clear the object without hitting the rear tire. this helps gain momemtum after the jump too. i cringe when i see people stop and walk over smaller logs. this technique will help you clear small to medium logs no problem. practice like Doc says is all it takes. start in your yard then try it on the trail.