[MrForbes]

To make the motor turn in precise increments, requires feedback and motor control. Use an encoder (a potentiometer will probably work and only costs $3 at Radio Shack) and a Victor or Jaguar instead of a Spike to drive it. Then you'll get to discover the joys of a PID loop, and getting the constants right.

The potentiometer is mounted so it moves as the motor moves, and connected to an analog input.