The best way to do this is to run something like Symantec Vault on the Exchange server with 30-60 day archiving to off-site storage. The data is archived, removed from the inbox, and is replaced by a link to the archived message.
However, I suspect that .pst files are in use on the local computer (very ugly). To back up the data one has only to back up the individual .pst files. The simplest way is to just copy them to a network share (WARNING: do not open a .pst that resides on a network share, it's a great way to corrupt and lose the file)