I agree with all the previous answers – programming, service and results will determine your success as well as increase retention. Service and programming are components that you control entirely, however it’s important to recognize that results are subject to the client’s perception. I find that the best incentive is to continually reinforce the ways in which they have improved – I do this both verbally and through demonstration. This includes objective measures such as weight loss but also less obvious progress such as increased endurance, strength and energy levels. I will continually comment on how X months, weeks, etc. ago they were not able to do x,y and z. This reinforces the perception that you are an effective trainer and that they are making progress. This might seem like an obvious incentive but you would be surprised how often clients don’t recognize or acknowledge these small and big wins outside of weight loss and increased weight loads!