I keep getting a notification on my mac that my start up disk is almost full. When I check whats taking up the most space (when I go to "storage" on about this mac) it says all the space is being taken up by other. How do I free space?

OS X 10.7 Lion/ 10.8 Mountain Lion / 10.9 Mavericks

1. Start up in Safe Mode

http://support.apple.com/kb/PH11212

2. Empty Trash.

http://support.apple.com/kb/PH10677

3. Delete "Recovered Messages", if any.

Hold the option key down and click "Go" menu in the Finder menu bar.

Select "Library" from the dropdown.

Library > Mail > V2 > Mailboxes

Delete "Recovered Messages", if any.

Empty Trash. Restart.

4. Repair Disk

Steps 1 through 7

http://support.apple.com/kb/PH5836

5. Disk space / Time Machine ?/ Local Snapshots

http://support.apple.com/kb/ht4878

6. Re-index Macintosh HD

System Preferences > Spotlight > Privacy

http://support.apple.com/kb/ht2409