Quick & dirty guide for myself :

virtualenv -p <python_version> <directory_name>

Cagatay-MacBook-Pro:virt cagataytengiz$ virtualenv -p /Library/Frameworks/Python.framework/Versions/3.3/bin/python3.3 p3  
Cagatay-MacBook-Pro:virt cagataytengiz$ cd p3/  
Cagatay-MacBook-Pro:p3 cagataytengiz$ source ./bin/activate

and then install required packages as :

pip install <package_name>