In an effort go get myself set up with an aerodynamic time trial package for my bike I purchased a new Rudy Project Syton SuperComp aero helmet last night (click the image to visit their site).
About 90% of the energy expended in pedaling a bicycle goes towards moving the body of the rider through the air. Futhermore, the drag that your body creates increases exponentially with speed, so as you go faster, aerodynamics become more and more important. At a basic level there are 3 things one can easily adjust that have a very large effect on drag:
1. Body position. Getting into a good position where your back is parallel to the ground and flat, elbows are in, chin is up, arms are preventing your chest from acting like an air damn, etc. makes a big difference. i.e. I need to buy a set of aero bars and work on my body position.
2. Head. Once you're doing well with your body position, your head is normally the next biggest source of drag. Hence the helmet.
3. Rolling resistance of wheels. Getting an aero set of wheels can make a difference. I don't see myself spending $1500 on a pair of wheels any time soon, so this one is probably going to have to wait. I might look into something used on Ebay if the right opportunity comes along.