Reducing drag is just like reducing weight on your race car, it allows you to go faster on the same amout of power. Other advantages of proper aerodynamic modifications to your car is the reduction of lift and the increase in downforce. This creates more stability on the straights and more traction in the corners. Just look at a Formula One car, wings and widgets everywhere, created with millions of dollars of wind tunnel testing. Well, a budget racer can use an old trick for the same kind of testing; tape some lengths of string around your car and see which way they move while driving. See this article on “tuft testing“. Or check out some of the many books on race car aerodynamics (a couple of them on my Books & Stuff page.)
Common devices include front spoilers or air dams to reduce drag and increase downforce (or at least reduce lift). Same for rear spoilers or wings. Some simple things work like lowering the front of your car which reduces the airflow beneath (less drag, less lift), or removing parts that stick out into the airflow (mirrors, latches, handles, etc).
Other ideas are lips to create low pressure areas behind them, and vortex generators to either smooth out airflow or reattach detached airflow.
Do they actually work? Only proper wind tunnel testing would confirm for sure. But it’s fun to think you’re doing some world class aero work on a budget racer’s nickel.