Gratuities are great, and your generosity is very appreciated! However, I consider massage to be an important part of a healthy lifestyle, just like exercising and eating healthy. In my opinion, it doesn't seem professional to accept money above my regular service fees.
In addition to this, sometimes people are quite relaxed and a little 'out of it' after their massage which may lead them to tip too much. Once this happens, it's common for people to feel as though they have set a standard for themselves and must always tip this same, very generous, amount - which leads them to not come back as often.
I would rather you skip the tip all together and spend that money on yourself, and more frequent massage.
If you are happy with the treatment provided, the best tip you can give is to tell your friends, family, and co-workers about Goertzen Massage Therapy!