Open a new shell and you should be golden. Never 'sudo gem install' again!
To install any Ruby, 'rvm install 1.9.2', for example. To switch Rubies, 'rvm 1.9.2'.
That's it! Pretty easy. I like to take two further steps, though: create a gemset per application, and set up .rvmrc files to switch to the right ruby automatically.
To do the gemset thing, 'rvm gemset create gemset_name'. You can switch to a specific gemset at the time you switch rubies with 'rvm 1.9.2@gemset_name', or after with 'rvm gemset use gemset_name'
Once you've got that going, make a '.rvmrc' file in the root of your project, and put in it the rvm command to switch to your ruby/gemset. "echo 'rvm 1.9.2@gemset_name' > .rvmrc" is probably the easiest way to do this. Then, when you switch to your project, you're in the correct ruby and gemset automatically. When you have 9 or 10 projects on 4 different Rubies, this helps a lot...