Safety stuff: Brakes, roll bar or 6 point cage, good helmet, fuel cell, fire system.
Lighten the car up all you can.
Track time. There is no substitute for time behind the wheel. If you think the car is two seconds quicker than you are, you've got a ways to go. As you get faster, the improvements get smaller and smaller, but that's what separates the good from the average. Quality instruction, from somebody who can help, goes a long way. A datalogger helps a lot also. Once you get to 80 -90%, most of the speed increases come from mental improvements, not mechanical ones.
John