Trimming is one of the essential skills to riding waves on a bodyboard. This technique gives you the speed and projection to ride a barrel and fly like the pros. The rider below is a great example of how your posture should be: eyes forward, elbows and hips on the board, inside rail engaged while centered over the board and your legs out of water (for less drag at higher speed).
A major problem that some people have while trimming is leaning too far forward on the upper inside rail. This is the main cause of unnecessary nose-diving and shoulder burying, spray in the face moments.
Stay upright and focused and maintain pressure on the inside rail without hindering your body placement over the center of the board. Move the board forward and backward to feel the difference in speed on different kinds of waves, but remember to keep your board centered with your body.