I clean mine while it's still hot, but not plugged in.
I scrape it with the plastic grease scraper first.
Then, I take a damp, warm washcloth and dribble water all down both halves, helping to drain off the excess grease. The water and grease goes into the grease pan. Then, I use a soapy washcloth and scrub both halves, folding the washcloth thickly so that I don't get burned. Since it can't be immersed, I have to do this on the countertop, but it works okay. I rinse it by wiping it with another damp, warm, washcloth until all the soapiness is gone.
**I've also aimed mine so that the dirty water drains directly in the sink, eliminating the need for the grease pan while washing. That works out well.
I let it air dry before putting it away.
Never use steel wool on your George Foreman. My sister's roommate in college ruined a George Foreman (belonging to my sister...grr...) that way.