Temporary FTP accounts?
I would create a different account for each client, locking them to their own directory. There is no way you want clients to see each other's work!
edit: just wanted to add, if you keep giving temp access out- it's going to annoy your repeat business. It's just as easy to setup perm accounts for them to use :)
- Don't do FTP. Use either SFTP or FTPS.
- Create unique accounts and directories for each customer.