When I come up to a trail like the one that you mentioned alot of the time I will slow down and pick a line that I know once I start to really pedal there's nothing to slow me down, then I'll stand up and lift my front tire over the root, then try to power right over the root with my back tire, while I'm doing this I try to sort of lunge the bike forward (not too noticibly, just enough to get it over the root) and this sort of pulls the rear over the root. Also, this is just common sense I guess but I try to keep my center of gravity right over the root or else you'll really slide.
This is just how I do it, I'm curious if there is a better/easier way.