It depends on the trainer. You need to assess their strengths and weaknesses. Not all trainers are salespeople but they can be great trainers. Then you might have some trainers who are great salesmen but not as good of a trainer. If you think these trainers are both, great trainers and salesmen, then you should offer a % of their sales as an incentive. If not, then you will need to do the PT sales or hire someone who can. If the trainer is good, he or she shouldn’t have problem selling their services. I have worked in places where some trainers were great closers and others who were not. Some trainers have the gift of first impression and they can close any deal, but then you have those who can’t.
Maybe you can offer a base salary and then give them a % for those sessions they renew. Having a sales person in charge of the initial sale might be a better deal and then let your trainers do their best to keep the clients buying sessions. If they can’t keep their clients, that means the client wasn’t happy with them and you will need to evaluate their performance at that point. I hope this helps.