When a deal with a prospective client is struck, convert their Prospect profile into a Client profile.
Select the profile in the Main tab of the Prospect window, click More... and select Move to Clients.
The profile and all accompanying table entries will be transferred into the Clients database, and all files will be moved into appropriate folders automatically.
If you no longer actively work with a client, you can convert their Client profile into a Prospect profile in the same manner from the Main tab of the Client window.
All the former client's Invoices, Jobs and other documentation will be safely stored in the Prospects database for reference purposes.
See also:
Adding/Editing Client/Prospect Profile