I actually do the opposite the rocknrollgirl does.

I have front suspension so I put my weight on my front wheel to compress, then lift and shift my weitght back to pop up the front wheel. Continue to pedal but you may have to shimmey the pedals to make sure they don't hit the log (now that's caused a lot of my tumbles!). Of course, easy said than done some days and depends on the size of the log. If the log is too big, I have been know to scour the landscape for a smaller log to pack in front of the fallen log to build a "ramp" to help me over. Gotta help 'em trail build sometimes!