Today I switched from RVM to rbenv. This ThoughtBot post was a great starting point.
I ran into a couple hitches. Here's how I got through it. Do this stuff before you start the ThoughtBot stuff.
brew doctor. I ran it a bunch of times and followed the steps until it was clean.
The hardest step was XCode. I was running 4.2.1 and brew wanted 4.6.3. I downloaded the new 4.6 version through the app store, but brew was still using my old standalone 4.2 install. Luckily when I opened the actual XCode app it prompted me to remove the old version! Perfect!
Also through the actual XCode app - I needed to open the Preferences, click "Downloads" and download the command line tools as prompted by brew doctor.
Smooth sailing after that! Just follow the ThoughtBot instructions.
Once rbenv is installed you can run
rbenv install --list to see the available versions.