On approach, it is important to shift your weight on the pedals to the balls of your feet, with your heels literally suspended off the back of the pedals so as to use the natural spring of your leg as an added measure of cushion.
Lighten the front end of the bike by shifting your upper body rearward, in some cases literally tug on the bars so as to wheelie the bike up on over the log. It is essential to try to hit the log at 90 degrees, especially as slippery, damp, or mossy surface will cause your tires to slip laterally upon dropping your weight.
Lastly be sure and prepare to remove a foot for balance (if you use clipless pedals, be sure and practice being able to detach your foot on the fly for situations like this) as you transition over the center (highest point) of the log.
It is critical to judge the height of the log before attempting to hop over it, as casing the pedal crank will not only hang you up, but also nullify the action of your suspension.
Successful navigation of logs is often dependant upon the judgment of the rider in knowing when getting off to carry is wiser than risking loss of balance or getting your bottom bracket or chain-rings caught up.