Yep agree with the other comments. You do need to keep your weight central, but sometimes you also need to shift your weight around. When an obtsacle comes along you need to select what is the best way to go over the obstacle without losing too much momentum.

There are three ways to go over an obstacle - 1. Rolling over it with both the front and rear wheel; 2. Lifting the front wheel and rolling with the rear wheel; and 3. Lifting both the front and rear wheel.

Obviously there are many factors that you must take into account when selecting the best option, including the height of the obstacle, the steepness of the climb, the stability of the dirt, and most importantly how much energy you have as a rider at that point in time.

On long technical climbs I find the easiest way to go up is to just keep pedalling and pick your way throught the rocks and roots and not bother about shifting your weight around too much. However, if the rocks or roots get big then obviously you will have to shift the weight around and throw your bike around a bit.

Hope that makes sense.