Whats the best way to version your ActionScript files?

Since using AS I’ve never found a good way of versioning my AS files, i.e. MyClass_01.as, MyClass_02.as, etc…At the minute I have a folder containing all my files needed for a project: .fla, .as, images, .xml and every time I want to keep a version I have to copy the whole folder and rename it a new version, this is fine for the whole project (well it does get heavy if I have lots of images) but for individual AS files I can’t keep a close enough track of changes…how does everyone do it? Do you do it at all? What’s best?