The Nature's Miracle works pretty well.

I also read somewhere that cats don't like the feel of aluminum foil on their feet, so if where they peed is out of the way, you can cover it with aluminum foil for a while after it has been cleaned. Mine peed under the bathroom counter, and I cleaned it (a million and a half times it seemed like), they peed again, cleaned it again and I put foil on top of the spot this time. It didn't keep them completely off of it (if they were spazy they would still run under there) but they haven't gone to the bathroom under there since...and I have long since gotten rid of the foil.