I think to a spa girl, looks have very little sway over how they act, at least in the beginning. This is a job to them, they aren't looking for a relationship.
However, if you are a regular, wealthy, AND good looking, they might have interest in starting a relationship. It's important that you are clean, tip well, and act reasonable. I also don't think being well hung is a benefit. Well hung guys, especially ones that like to pound away, are apt to leave them really sore. After you're done, depending on the time of day, they may need to service 10 more guys.
I think the only time you can make a big impression is if you're their "type." This happened to me when I first started. I had just come back from backpacking for a year and was severely underweight. One provider absolutely loved this and gave me constant compliments on how I looked and invited me over when the spa was closed and the other girls were gone. When I visited she was in full blown porn star lingerie, but I showed up at random once and she was in sweats and a tank top. Eventually I got back to my normal weight (BMI ~20) and she lost interest. Providers have fetishes too.