I've found a simple way for me - I made a "cradle" in which I could place them upside down and then used a little white spirit on a cotton bud to get the worst of the black crud off, followed by a dry cotton bud to soak up any excess and prevent damage to the plastic.

Keeping the rails clean first, however, is an absolute must for me. I require the models to move very precisely over short lengths of track for filming, and keeping the rails spotless on their heads solves most problems of electricity pickup. I find the wheels pick up less crud as a result, too.

'Course, I may be teaching you to suck eggs here!

If you have any old/tired/well used track rubbers knocking about, cut out the largest circle of rubber you can and put a hole through the middle. Attach to a long screw with some washers and pair of nuts to lock the rubber tight and put the free end of the screw in your hand held Dremel/hobby drill. You've now got a track rubber whizzy wheel. Take care, especially where sanding pipes are near the wheel to be cleaned.

Cradle? Plonk the loco upside down in the four-foot of a length of spare, straight track with some thin foam as paint protection.

A pair of wires from a track feed occasionally touched on the wheels of said loco will negate the need to keep lifting the loco off and onto your cradle.

C6T.

Trix wheel cleaners. Wire brushes set in a plastic case. Sit it on rails, sit loco on it, hold gently, apply track power. Sorted.

http://www.amazon.com/Trix-Locomotive-Wheel-Cleaning-Brush/dp/B0000WRG64

(only works for driven wheels, mind)

I'll agree with Jamie, for power units the best I’ve used are the Trix cleaners. Their quick and are very easy to use, brilliant for exhibitions. In the past I’ve used glass fibre brushes which are effective but time consuming.

Trix also do n gauge versions and there's no problems using them with DCC.