The solution for this is just to use virtual environments, so that each one is isolated from the others. To do this, there is a tool virtualenv, easily installed by easy_install virtualenv, and even more easily used by virtualenv <projectNameDir>. This way, nothing is stored on the common site-packages, and files under <projectNameDir> have their own site-packages and are isolated, so you can have many environments in the same server.
Note: When you are inside <projectNameDir> and you run your commands like 'python manegy.py ...' make sure to use python (and any other tools) found at bin folder of your virtual environment. So you will end up running commands like
../bin/python manage.py syncdb