This article talks about how to maintain control of your bike going over different types of obstacles that you might see on biking trails.
When going over logs, it is important to have a good speed coming at the log. If not, you're front wheel will likely not make it over or your back wheel will not have enough momentum to climb the log. Once, you're close enough to the log, jerk up on the handlebars while your weight is shifted back, then shift your weight forward so that the back wheel can climb it. Do not apply the brakes or you will probably not make it over. When traversing rocks, you want to keep a steady speed as well, with lots of weight on your peddles. Stay seated to keep the centre of gravity low and your bike more steady. Try to stay on the smooth and more gradually sloped part of the rocks rather than the steeper and bumpier parts. Going through water and mud are pretty similar; keep pedalling while you're in the liquid and go straight to prevent slippage. Scott also brings up a good point in his article; be courteous and try not to go around the mud, because it just makes the mud spread out more. When driving a car, wet leaves are actually more slippery than then ice! Generally, ice is more dangerous however on a bike, but when biking over leaves, especially when they're wet, it is important to not apply the brakes too hard or make any sudden turns because you could easily slide off the path. These are all very useful tricks, and if executed properly, you will rarely wipeout.
S.Adams. How to handle different obstacles on the bike. http://www.active.com/mountain-biking/articles/how-to-handle-different-obstacles-on-the-bike