Scott Chacon (@schacon) has written a fantastic post regarding how we develop GitHub using GitHub itself. Do yourself a favor and head over to his blog:

http://scottchacon.com/2011/08/31/github-flow.html

If there’s only one thing you take away from his post, understand that Pull Requests are not only useful when your code is ready for review, but can also be used as an extremely effective collaboration tool while features are being developed.