How do i ride MTB on a balance beam?

i've ridden mtb for a longg time now, i cant do a wheelie for more then 10seconds, and i cant ride on the curb in a straight line like i want to. i see in vids they have a tree trunk thats all skinny and they ride over it, i was wondering if theres any vid/tips to teach me how to do that.. or just practice it [ill do that anyways]