You need version control system. First choice is SVN. It has good eclipse plugin (http://subclipse.tigris.org/). You are able to find a free svn hosing (google code, source forge, open svn, assembla). SVN is a centralized system - you need a server.
New trends says that distributed version control systems are better. You don't need a server. Three most popular are Git, Mercurial and Bazaar. They have explorer plugins ( Tortoise...) and maybe eclipse plugins. They work with servers too but shared disk is good too. Google code provides Mercurial server, but you are able to find free hosing.
In Flex projects remember not to version .settings folder and files :
.actionScriptProperties
.flexProperties
.project