I have a lot of Flash Builder workspaces so Eclipse’s default behavior of remembering only the last five workspaces has always been annoying to me. I have projects for various clients and I like to keep all of the files for each project together. I also have several Flex/ActionScript books which come with code samples. To make it easier to track the specific files for a given project and to keep from getting one huge cluttered workspace I create a separate workspaces.
Unfortunately, Flash Builder/Eclipse defaults to remembering only the last five workspaces so I often end up drilling down through my directories manually to find a workspace. However, I just discovered that it also offers the option to increase that number. I just increased mine to 15 so that should keep me from searching around so much in the future.
There you can select the number of workspaces to remember as well as the option to ask you which workspace you want to use every time you start Flash Builder.