Most spas where you spend the night, including resort spas, hotel and destination or health spas, add a spa tip or "service fee" of 18-20% onto the massage or facial, so you don't have to worry about it. However, if you think you received exceptional service, you can give the therapist additional money in the treatment room.
If you offer the therapist money and a tip is already being added to your bill, they should inform you of it before accepting the money. If you didn't know, feel free to keep the money.
Some day spas add a service fee, but many do not. You can either offer it directly to the therapist in cash, or add it to your bill. A few spas leave envelopes in the room to encourage tipping.
There are a few exceptions to the spa tipping 15-20% guideline. Some spas, especially medical spas, don't allow tipping.
If you're getting a bargain price, like $50 from a chain like Massage Envy, you should tip $15 or $20, especially if you like the therapist.
If you're getting a promotional rate like $50 during Spa Week, you should tip on the full value of the service.
There really needs to be gross negligence such as rudeness or sexual advances on the part of the therapist to not tip at all at. Let them know at the front desk what happened, so management can take care of it.
If the person is doing their best, but you just don't like their style, go ahead and tip. Just don't book with them again.
If the therapist has a private practice, they get to keep the whole amount of the service. Tipping is not expected, but it is still appreciated. You can also take into account how much the therapist is charging. If they're charging a relatively low rate like $60 or $65, you might tip. If they're charging $100 or $120, that is already a good fee.
If you go to the same therapist regularly, it's also nice to do a little something at Christmas to show your appreciation for their work year-round.