It may be controversial, but contracts are a false sense of security. Contracts do not prevent members from leaving, and it prevents them from coming back. Yes, you may be able to send them to collections or make them pay a large cancellation fee and get a few more bucks from them, but in the end, you have burned the bridge. Life happens, and sometimes people quit for various reasons - mostly, it's financial. If they can't pay you now, they won't pay if you send them to collections, and the chances of them returning are zero. There will always be members quitting - it's inevitable - sometimes it's because they are not happy at your gym for whatever reason. That's hard to swallow, but you can't please everyone. It could be something about their experience at your gym, in which case it's a great learning experience. If it's something you could have done better, try to see it from their perspective. Have empathy for their point of view - put yourself in their shoes to understand why they want to quit and try to come up with a solution together. Let them go if you can not come up with a satisfactory solution.
On the other hand, it could be that they are hard to please, and you don't want that type of person anyway. Most of the time, it's financial. If you approach it with understanding, they are much more likely to return when their circumstances improve.
We have a 90-95% monthly retention rate with no contracts. We earn our member's loyalty daily through the experience we create and the atmosphere we cultivate.
Agreements are a better solution. You can outline what you will provide in exchange for the membership price and what is expected of the member requiring a 30-day written notice to cancel, etc.