Laying the tip out on the nightstand is not a bad idea. But if you wish, you can just explain to the girl that this money is for her only if she delivers very good servce. And once again, unless it is extraordnary service, I would only put out $100.
While I stated that you need to be nice to the girls (and I'm very glad that you are), there is no need to be foolish.
Better luck next time.
LexLuther