On my motor, I pulled off the ends, removed the rotor (gently, don't let it bang around as you pull it out!) then removed the bolts that hold in the field coil.
I hung the field coil from a tree and used a can of spray-on electrical insulator to recoat the insulation. It looks like green spray paint, but is actually a good quality insulator. I got it from a friend, but I think a can of it only costs like ten bucks.
I also inspected the bearings in the motor, which were fine and did not need replacement.
I visited a local forklift motor repair shop, where I got new brushes for the motor. Half the total cost of my motor was new brushes! ($50 for the motor, $50 for the new brushes!)
While I was there, I told them what I was doing and asked for advice. The guys showed me the machine they have for re-cutting the grooves on the commutators, but that you could more or less do the same thing by hand with a pick or hacksaw blade.
They also gave me a "Commutator Stick" which looks like a cross between a nail file and a lipstick. It's an abrasive material which you simply press against the commutator as it spins to smooth it out.
The commutator was cleaned up by a friend while he had my motor for helping design the adapter plate. He had a lathe, so he just zinged off the tinniest bit of the commutator to reveal fresh, clean copper.
I cleaned out the commutor grooves with a pick.
I ran the motor on my shop floor at 12V. With the new brushes, the clean commutator, and the grooves cleaned out, there was still a tiny bit of visible arcing and a little high pitched whine. (The whine was the motor, not me complaining!)
With the motor spinning, I gently pressed the commutator stick in there, rubbing it across the commutator. Instantly, the arcing and high pitch noise went away! I don't know what's in a commutator stick, but it sure worked great!
That commutator guide that Ryland posted looks like some good information.
Another place you might want to look is at a thread on DIY Electric car, started by EV Motor Rock Star, Jim Husted (username: Hi-Torque Electric.) He tells you how to pick a good forklift motor to convert to EV use.
Using a forklift motor, and choosing a good one - DIY Electric Car Forums