While you actually have more control if you do this while standing, I find it easier to do from a sitting position (starting out).
First, get used to using just one finger for the clutch. That way you get to hold on to the bars with the other three. I rode MX for 20+ years with two fingers for the clutch until I got a street bike that was (more than) capable of wheelies. I found that the most controllable way to wheelie the street bike was to clutch it up in 2nd with just one finger on the clutch. Since riding on the street is a much more relaxed ride with less things happening, I finally got into the habit of always riding with one finger on the clutch lever and only using that one finger.
Second, out on flat ground practice clutching-up the bike until you are completely comfortable with doing. For quite some time, limit it to merely getting the front 6 inches off the ground and riding it out until you need to shift. At 6" that won't be too far.
Third, when you actually do go to cross the log, don't try to carry the front wheel over it, but rather try to get the front wheel to hit on the top half of the log. If you wheelie the front wheel completely over the log, when the back tire finally hits it, it will throw the front down violently, possibluy even sending you over the bars.
Fourth, once you have hit the log, stand and allow the bike to rock backward then forward under you. If you try to stay sitting, it will throw you off the seat. As you hit it with the front elevated, you stand up and forward. As the front drops and the rear rises, you move rearward to maintain your basic body position (a related to the ground) throughout the process. After the rear comes back down you should move forward and start accelerating.
You don't see trials guys sit down because they don't have anywhere to sit! Well...that and you really do have more control standing. When sitting you stand the chance of still getting thrown over the bars if you don't stand quick enough when you cross something like a log or short, steep kicker jump.
I posted it before, but my clumsy way of log crossing:
http://themxtrack.com/Barrel.wmv