I would also recommend installing [virtualenvwrapper](http://virtualenvwrapper.readthedocs.org/en/latest/) which "wraps" around virtualenv and makes commands easier. For instance, to activate a virtualenv, you would just do `$ workon my_venv`; to create one, you would do `$ mkvirtualenv my_venv` and to deactivate, just type `$ deactivate`. 

You can install it via easy_install: `$ sudo easy_install virtualenvwrapper` then add the following lines to ~/.bashrc or ~/.bash_profile or ~/.profile (whichever you have):

    export WORKON_HOME=$HOME/.virtualenvs
    export PROJECT_HOME=$HOME/Devel
    source /usr/local/bin/virtualenvwrapper.sh

Then start a new terminal window.