NASM
Use common sense. Dont have overweight 58yr old ladies doing squats and "core" work.
What works for you, works for them = BASICS
be passionate and sincere with clients as it is they who determine how good of a trainer you are.
Great trainers have a nice client base due to word of mouth and other gym members seeing results happening.
Practice what you preach, Dont be a personal trainer if your gonna come in hungover, or eat candy and junk in the gym. Shit just doesnt fly, you have to look the part.