I charge clients in advance and require full payment of services. I offer a small discount the higher the number of sessions. However, I am also flexible, and have allowed clients to pay 50% upfront and the other 50% at an allotted time. I have my own in-home business and it works for me (most of my clients are long term and I’ve never had an issue with payment).
Payment in advance is pretty standard–although I know for some clients it can sometimes be overwhelming. Definitely do your research on the trainer you are interested in and make sure you are comfortable.