start small...get comfy...go a bit bigger!

i do the compress fork then lift as well...
compress...rock back with bike while lifting the front wheel over or on the TOP of the log..then rock back forward (gently) to lift the rear wheel over the log. i cannot technically do a bunny hop (which is the above described whilst getting the wheels off the ground) but i do get enough umphf to get over most logs! the ones that get me are big enough to get the chainring so it is best i clip out for those anyways
remember to have pedals level as well so they do not hit...try to pedal as you go over the log in order to increase umphf power....i usually set up my right foot (dominant power leg) to be around 1-2 o'clock position on the front wheel lift and push on the pedal as i am going over the log.

i rode for months on a trail that had 3 logs on it...they were buried for the most part but i would imagine them as larger than they were and would use this technique until i was flying over them without touching...then i would go to the next larger logs and work on them...so now every downed branch...twig...snake...i can umphf my way over. i still walk logs that look like they have taken out a few chainrings but my bottombracket sits a wee bit lower and i always take the safe side...

now if you are talking log overs that are "built up" layers of logs forming an A-frame type pile...same technique but you must be ready with that pedal stroke to power you over the top...and yes speed is your friend ...but you do not have to bonzai thru it...just enough power to get you up and over. those do take a bit more hutzpa...just make sure they are stable and you know what is on the other side (ramp or drop)

a spotter is always a good way to start....it helps your confidence thru the first couple tries! and remember..."tuck and roll"

good luck...and rubber side down!!!