Get a second instance of filemaker server for development and create some scripting that transfers data from production to development as part of your deployment process. Thats the easy part. the hard part is going to be detecting and resolving inconsistencies in schema between production and development for the data transfer process.. i.e. field or table that exists in production and does not exist in development or vice versa.
Base Elements is a vital part of the process for me...
The process goes something like..
1. Development env files are deemed ready for deployment to production.
2. Run dev env files DDR thru base elements and resolve any issues.
3. Lock users out of prod env files.
4. Transfer data from prod to dev.
5. Test dev to see that everything is still working correctly.
6. Move files from dev to prod.
7. Test prod files.
8. Allow users.
The paid tool RefreshFM was created exactly for this purpose.
It's made by Goya, the same people who produce Base Elements. Goya are (excellent) developers themselves, and originally they created FreshFM (was called UpdateManager) for their own internal use, and it later grew into a commercial product for other developers to use.