A good start when learnign to ride light is, before you hit a bump or pothole or something, simply stand up and put as little weight as possible on the bars with your hands... the bike should be able to freely rock fore and aft as the wheels go over the obstacle, and no damage will result.

The slightly more advanced version: While the front wheel rolls over the obstacle, the bottom bracket (and by extension most of your body weight) will rise about 1/3 the distance the wheel rises. Then, as the front wheel rolls down the back side, switch your weight more to your hands and unweight your pedals to allow the back wheel to ride up and over.

The trick is to keep riding while doing this, and, IMHO, it isnt a very difficult trick, but it will be slightly more physically demanding at first while you get used to it. It is also much harder to do when you are exhaused at the end of a long ride, so you really have to force yourself then.

I have known many people who have broken multiple seatposts, saddles, wheels, suspension forks, etc... usually teenagers... who brag about it as if it means they take "sicker" jumps or ride harder - the truth is the opposite - they have not learned to ride smoothly, and their bikes pay the price.