Normally I would tell you to write it into the profile data of the player (just like the vanilla bank script does) but that seems to not stick for some players. Without that there really isn't much of what you can do when the script is running on a client / listen server.
IMHO without reinventing the wheel you could try the following approaches:
You could write the data in a dvar so it will at least survive until the game is restarted.
You could persist it through iw4madmin or feds gsc-util plugin when dealing with servers.
You could also split the functionality into 2 files, one for the logic and one for persisting the data where you could provide different versions of the later depending on what method should be used to store the data.
