How can I make my bike more aerodynamic?

Assuming you are keeping your existing bike frame, the top two ways to decrease aerodynamic drag are to ensure you are comfortable riding in an aerodynamic position, and upgrading your wheel set and using road tires. Improving body positioning will have the most benefit at the lowest cost. Because wheel spokes are continuously cutting through air as you ride, they induce drag. Spokes on wheels with less aerodynamic drag allow for the wheels to roll with less resistance. Aerodynamic wheels are generally lighter which also decreases rolling drag.