There are no "magical" commands to shrink the database. The database file (EDB) will only shrink if you perform an offline degfragmentation, and even then it won't shrink unless the database had white-space (free space) in the file to begin with.
Assuming you are doing backups with an Exchange-aware backup, your database doesn't have any significant quantity of white space in it, and the database file really is approaching 250GB, there's not a lot that you can do other than add storage or get users to delete a sufficient quantity of items (and perform a backup so that those items are actually flushed from the store) in order to create white space in the database file to arrest the growth of the database file. (You can find your white space by looking for event ID 1221 in your Application Log, from event source "MSExchangeIS Mailbox Store").
My guess lies with the other posters' answers, though. You're probably building up database transaction logs (do you see many, many gigabytes of ".LOG" files in your Exchange database directory-- \Program Files\Microsoft\Exchange Server\Mailbox, by default) and you're not doing proper backups. If you're not using an Exchange aware backup you're likely not going to be able to recover your server in the event of a fault condition, and you're going to have disk space exhaustion like you're seeing.
(It is theoretically possible to enable circular logging for a storage group and stop transaction log growth, as well, but you're sacraficing recovery capabilities if you do that.)