This stuff is the best.
http://urine-offstore.com/
Lexi has not had an accident in about 6-8 weeks but I could still smell urine. I used this a week ago and I can't beleive the difference. Well yesterday was the big test, my husband cleaned the cat box and decided to spray this stuff on the empty cat box and let it air dry. OMGosh, no more smell at all.
It's a little pricey but so worth it. I bought the small spray bottle a week ago and my husband just went this morning to buy the gallon...