The Lone Wolf said:
Ok steep climbs are kind of my specialty. You need to lean forward and down, get down so far your head is about 6 inches from the handlebar. Then you need to find the sweet spot where your front end stays down but your back stays turning. Lean forward and back to achieve this. If your front end comes up lean forward more. If your back tire comes loose lean back. Spin at about 80 RPM and try to make even strokes all the way around. If you thrash on the pedals the back tire will break loose. Practice a lot and you will get it down.
Like The Lone Wolf says, there are 2 parts that you need to practice:
1. Balance front to back. Most people think of balancing side to side to ride a bike, but its just as important to have balance front to back when riding technical trails. You need to find the spot that works best for you, which you can only do by trying again and again.
2. Steady but firm pedal stroke. You need to keep the pedals moving smoothly so that the rear tire does not jerk and break loose. You need to have a hard enough gear so that there is tension all the way around the pedal stroke, but easy enough that you can keep pedaling smoothly. The hardest part for people to learn, is keeping the pedals spinning even when the tire does slip or bounce over a root or rock. Most begineers just stop when the back tire slips, but if you just keep spinning and maintain balance, you can usually recover and keep going. Remember that momentum is your friend!
Its best to find a short steep hill and spend an hour every now and then just going up and down. You'll get the hang of it soon!
Mike