If you want FS, tell the lady you want FS with the first words out of your mouth after the initial, common greeting. Determine the price you want to pay and tell her that is what you are willing to pay. Then, the girl determines if she is willing to accomodate your service and price request.
Girl: How long do you want to stay baby? What do you want sweetie?
You: I want Full Service.
Girl: Full service is $ xxx (dollars)
You: I have $ xxx (dollars) on me. Let's have fun beautiful girl (say it with a smile and in a friendly way).
She will know what her lowest price is. If you are at or above her lowest price, she will accept. If you aren't at or above her lowest price, she will either refuse or negotiate up. If you truly gave her your highest price and if she refuses your offer, walk away.
Don't go in to the spa and low-ball the girl in order to save $20 - $40. Give her your highest price once and if she refuses, go somewhere else. She will be sucking and fucking you if she accepts, so no point in trying to low-ball her and put her in a sour mood, just to save $20. She won't give you her best performance in that scenario.
After 5 - 10 attempts and sessions, you will know what the market price is for the type of girl you meet. And you will be at the market price. And she will know it. And she will know you are serious and not wasting her time. Give the impression of experience and knowledge and she will pick up on this and most of the time agree with your price request.
Know the market rate. Pay the market rate. Enjoy yourself.