Updated: July 12, 2013
The instructions below are just quick references if you know what you're doing.
If you are installing RVM, Ruby, and Rails for the first time, I'd recommend following Daniel Kehoe's instructions:
http://railsapps.github.io/installing-rails.html
https://developer.apple.com/downloads/
a. Homebrew Install:
$ ruby -e "$(curl -fsSL https://raw.github.com/mxcl/homebrew/go)"
$ brew doctor
b. MacPorts Install via pkg:
https://distfiles.macports.org/MacPorts/MacPorts-2.1.3-10.8-MountainLion.pkg
$ cd ~
$ echo 'gem: --no-rdoc --no-ri' >> ~/.gemrc
$ \curl -L https://get.rvm.io | bash -s stable --autolibs=enable --ruby
$ source $HOME/.rvm/scripts/rvm
$ rvm --default 2.0.0-p247
$ gem install rails
$ gem install bundler
$ rvm install 1.9.3
$ rvm use 1.9.3
$ gem install rails
$ gem install bundler
##Additional References:
$ rvm use 1.9.3
$ rvm default
Single user:
$ \curl -L https://get.rvm.io | bash -s stable --autolibs=homebrew --rails
Multi-user:
$ \curl -L https://get.rvm.io | sudo bash -s stable --autolibs=homebrew --add-to-rvm-group $USER --rails
$ rvm --debug requirements ruby
$ rvm --debug use --install ruby
$ rvm get stable
$ rvm install 1.9.3
$ rvm use 1.9.3
$ rvm rubygems latest
$ rvm install 2.0.0
$ rvm use 2.0.0
$ rvm rubygems latest
https://rvm.io/rvm/autolibs
Homebrew
Github Issue: Install fails on 10.8.3 without Xcode installed