Django version is 1.4. I had read the official document, and googled my problem.
first I had followed the official document Managing static files added
I realize this has already been answered but I wanted to provide another answer in the event that STATIC_URL still renders as empty even when using RequestContext.
If you are running the development server remember to start the server with the insecure flag to have the server serve your static files:
python manage.py runserver --insecure