Use an IR LED and IR sensor to add position feedback to the motors. You can build it yourself or a similar servo wheel encoder kit built by Nubotics is available from Acroname. Some sensor modules are available that have both the IR LED and IR sensor mounted in a single plastic case. For reflective sensors, mark the wheels with radial black paint stripes or black drafting tape and count the pulses from the IR sensor to determine movement of the wheel. Another option would be to draw the radial stripes using a PC drawing program and print it on clear adhesive labels made for laser printers. The labels could then be placed on the flat side of the wheel. If a transmissive sensor arrangement is used, holes can be drilled in the main wheel or a second smaller slotted wheel could be attached to the servo output shaft that periodically interrupts the IR light beam from the LED to the sensor. In this case, the LED and sensor are mounted on opposite sides of the wheel. This same optical sensing technique is used in many mice to detect movement of the mouse ball. Use the position feedback to implement more accurate variable speed and position control for the motors.