By convention I use an rvm gemset per project - named after the directory that the project lives in.
For example src/rails/foo would have gemset foo
Since I switch a lot between machines - I always end up having to go check if the gemset exists and create it if not.
A small bash function (dump it in .bash_profile) and then when in the projects home dir you can just run rvmgo (or if you want a different ruby then e.g. rvmgo 1.8.7) and it will switch to the correct version/gemset creating the gemset if needed.
Note - it will not install a missing ruby.
