This could be useful. (How to reduce your PSD files size using Photoshop)

Apparently if you add an all white layer to the top of your photoshop files it reduces their file size by around 20%. The reason for this has something to do with not having to store the information used for thumbnails and previewing.

Here's Some More Info On It:
http://idletogether.com/how-to-reduce-your-psd-files-size-using-photoshop/

Pretty Sweet.

Edit:
Just tried it, it works! Wahoooo.