a teardrop helmet and body position. Here are some lab notes from the boys at the MIT wind tunnel:
80% of all the power a cyclist outputs goes into overcoming air resistance (the other 20% goes into rolling resistance, accelerating the rider and bike, etc.).
Roughly 75-80% of aerodynamic drag is due to the rider and only 20-25% is due to the equipment (bike, wheels, helmet, etc.).
A common misconception about equipment is that of the order of aerodynamic importance for aero-gear. Most think that the frame matters the most, wheels next, and helmet last. Some even think that the components come before the helmet. In reality, a well designed aero-helmet will save you more time (power) than anything else. The drag difference between a vented road helmet and an aero-helmet is 2-4 times larger than the difference between a good aero-wheelset and a 32-spoked wheelset.
In other words, if you want to go faster on the bike without investing in a TT bike, buy an aerohelmet (and learn to live with the ridicule of wearing one) and refine your body position.