As in all services-related businesses, it is common practice to tip the person who has done a good job to the satisfaction of the customer. The rule of thumb in day spas is 10% – 20% of the cost of service. Spas usually provide an envelope at the front desk when checking out for you to place the gratuity into it with the therapist’s name. You do not have to carry money around with you and worry about this while enjoying the spa facilities.