This Wiki page discusses strategies for dealing with historical data. It was originally inspired by Paul Stork's response at http://social.technet.microsoft.com/Forums/en-US/sharepoint2010customization/thread/31cf9e61-4faf-4832-aefc-a110d7d93ad2.

Finally, the most common option: eventually archive the items to a different site collection in a different content database if you need to save space.