A common question of how much and when to tip a massage therapist is commonly asked at a massage parlor. As a general rule, you don't always need to tip for a massage, but it is a good way to express your appreciation for a job well done. Generally speaking, when you get a massage at a spa or similar place, it's appropriate to leave a 15% - 20% tip depending on your experience.
If you receive a massage in a medical facility (similar to a place where you would receive physical therapy), you generally don't need to tip, but it's ok to ask if you think other people do.
Massage therapists are typically paid more than waiters or waitresses (due to experience and educational training) so it's not as critical that tips are made, but I'm sure with anyone you meet in life, it's always nice to get a bonus for a job well done.