I just now got around to installing RVM on my Mac. I wish I had done it a lot sooner!
RVM let's you install multiple Ruby versions side by side. You can easily install a new ruby version just by typing:
rvm install ruby-1.8.7-p160
rvm install 1.9.2-preview1
And you can quickly switch between versions by typing this:
Note that you will need to install gems independantly in each of your ruby environments. But they make it easy with gemsets. Also note, that if you want to preserve your custom ENV settings on your Mac, you should place the bash source line into your .profile file and not to .bashrc as it notes.