Bunny hops, J-hops, coaster wheelies, etc all root from simple transfers in weight. Clipless pedals have the tendency to allow you to "cheat". Learning these maneuvers on flats will force you to learn the correct way.
I was going to recommend James Wilson but he has already posted some good info. As a pro racer with 21 years of mountain bike riding, training and racing I haven’t met anyone with the grasp of how to strength train for mountain biking as James has. Definitely check out his programs.