WPMU now has a built in autosave feature that automatically saves when the author is editing a post. The main disadvantage to the feature is the frequency at which it kicks in. While some may type up their posts elsewhere, I actually type my posts into the WPMU editor. By default, the autosave kicks in every minute and on my computer/connection averages about 3 seconds to complete. I have also had issues with a manual save giving a WSOD if an autosave was already in progress. At the 3 second average that means there is a 5% chance that a manual save will intersect an autosave.
You can alter the autosave behaviour by adding a couple defines to your wp-config.php:
The autosave interval is in seconds. An interval of 300 is 5 minutes. Unless there was a compelling reason to do so, I would not have the autosave interval set to less than 5 minutes. 10 minutes is still a fairly reasonable interval.
A second feature that has been added is post revisions. A user can access these via the bottom of the write post area. If post revisions go unmanaged, they are likely to significantly swell the size of your database.
With this define, I am telling WPMU to only keep 3 revisions. When the 4th revision is saved, the first is purged off. Alternatively, post revisions can be disabled with the following define: