Side bridges are used to work the oblique muscles in the same manner that they are worked in everyday life. The oblique muscles are used to stabilize the spine.
Lie down on your side on the floor, and use your forearm to hold up your body. Your arm should make a 90 degree angle and your forearm should be flat on the floor. Your arm that is not on the floor should be held on your hip. Now, use your oblique muscles to hold your body up in a straight line. Your body should form a straight line from your feet all the way up to your head. Hold this position for as long as you can and then switch sides. Repeat this movement for 3-5 sets on each side.
As you master this movement you should either do more sets or hold the position longer. Try to hold your body up for 60 seconds on each side.