negotiate. I never do.  

 
I leave the rolled towel rolled and leave the tip on the side table. Nothing is said. The provider either leaves the towel off and isn’t shy about where she massages me, or is free to place the towel where she wants it. In the latter case, I try someplace else next time. That doesn’t happen very often.

....but in the few times that I've had to it was rather simple and reasonable. If I'm visiting a new spa I'll leave her planned tip peeking out from under my phone on a nearby table to let the her know that I'm interested in more than just a massage and that hopefully the amount will be acceptable.