[Joel J]

Don't use it. You can factor out the magnetic field induced by your robot, and you can calibrate for the offset caused by being indoors (these two things take work, however), but you can't account for random ferrous (would stick to a magnet) bodies in motion.

GPS would be a major help to you, if you were operating primarily outdoors.

Joe J. has a good point, though: you need some sort of stable reference to "zero out" your position error every once in a while.