I’m going to take the approach of just considering the retention part (and not how much they pay). I agree with Sue and Harris that you will want to keep the client challenged and interested by mixing things up a bit so they don’t hit a plateau. It definitely helps to keep them motivated. However, you will also want to keep some things the same to show continuous improvement and progress. Clients can stay motivated when they see how far they’ve progressed. They will want to renew with you if they are getting what they need from the sessions–whether that is a great, tough workout, accountability, or training for a big race.
Once you’ve succeeded, the next thing you can do is establish a new set of goals to keep them motivated. In addition, it important that you make a connection with that client. Clients know if you have their best interests at heart.
Good luck to you.