does the motor body have flats on it? or is it perfectly round? if there the round type, Take the motor apart, so u have the motor can with the magnets and clips inside. mark the position of the magnets, then cut a plate of metal or wood that fits between the aposing magnet retaining clips, twist the metal or wood while holding the can in a vice..

Rotating the magnets will alter the speed of the motor for forwards or reverse depending on the direction you twist it.

You'll have to just play about with it, it wont need much of a movement of the magnets to make a big change in the speed of the motor. about 10degree's or so and you will notice the speed differance, (listen for the pitch of the motor while running to get a ruff idea of how close your getting)

Alternatively a gyro will work and be easyer yes.

this is worth doing aswell http://www.rrc.dannysrobots.co.uk/07-1.jpg
http://www.rrc.dannysrobots.co.uk/07-2.jpg