Glute Flex's:

Lie on your back and tense your buttocks. Flex your buttocks 5 times with a 3-second delay before unflexing. If you feel even a slight burn, then you are very out of shape. This is where most of your thrusting force comes from where sex is concerned. Your buttock holds most of your thrusting power and stamina, with your abdominal being second in line. Here's the Flexing exercise from start to finish:

Lie on your back, and make sure you are on a floor, or firm surface so that you have the appropriate resistance to force against (i.e., the carpet, floor, etc. Your bed is far to giving and will not work for this exercise.).