What do you think is the most important aspect to keep retention high?

In no particular order:

1. Deliver what you have promised.
2. Always get feedback from your clients/customers.
3. Keep an open mind.
4. If you own a gym make sure it stays clean and safe.
5. Find ways to stay ahead of your competition.
6. Great customer service and loyalty keeps your clients/customers around.
7. Getting the results they wanted will make them stick around longer.