Here's my bit of advice:

Ride your bike around your house/apartment/neighborhood slowly and hop up and off every curb you find.

The thing you're probably not getting the hang of is body position. The correct body position is the KEY for clearing logs, roots, rocks, etc. and it's generally not something intuititive. Set up a small, easy, obstacle course in your yard or a local park. Nothing elaborate, just a few curbs to hop up, sticks to cross, sidewalks to U-turn on, etc.

As you ride down a curb, feel how your body is thrust foward by the drop. Then figure out how to counter that thrust by pushing your body to the rear. Eventually, you'll feel the bike going over the drop BEFORE your body. Feel that once and you're on your way.

I learned to clean logs in my driveway by riding over my curb a few hundred times.