Illustrator File Damaged And Could Not Be Repaired

its been coming more and more common for my files to become damaged and not be able to open for me. It started with files that had an insane number of veticies. So that was sort of undersandable. Now it just ruined 20 hours worth of layout work. The file wasnt that involved but it was exactly how the client wanted it. I have a purchased version of cs2 and mac osx 10.4. If you do not have an answer for how to repair the file can you atleast tell me how to prevent this from continuing in the future.