I know I've blogged about this before, but it bears repeating:
Don't lie. If you have no intention of tipping, just walk away. Don't tell the front desk, "Oh, I left my wallet/purse/ATM card upstairs in my room, I'll come back with the tip. What time do y'all close?"
I can count on the fingers of one hand how many times a client who's said something like that actually followed through, and that's if I amputated three fingers and a thumb.