In the city, have your hotel call you one. Just don't take a gypsy cab that you flag down on the street. Dress at the spas is casual, and shorts are okay for men and women. A sweater and light jacket might be needed in winter in Avandaro, and rarely in Cuernavaca. Don't even think about taking your high heels.
Flat shoes work best.
As always, if you have specific questions, we are here to answer them for you. That is what sets us apart from our competition. We know Mexico and Belize and have been to the spas we represent.