I think that one aspect to consider when it comes to luxury is taking the time. Depending on the treatment of course, but something like a facial or massage could be given for longer. Of course that isn't always practical when it comes to making money!
What about all the little things too like the kind of refreshments you provide or quality of towels etc that you use...?
To me, luxury also gives a picture of serenity. I find a facial or something like that, isn't as good if it's at a busy salon where people are talking etc. I'd rather pay more and go somewhere more fitting to that type of treatment like a day spa.
I guess with luxury comes spending more money. But that can be okay if you are able to charge higher prices for the treatments.