After you forked a project and merged back to the original, you can simply delete the forked project.
If you are like me, I always forgot to delete the forked project. When I need to contribute again, I need to bring my forked project up to date with the original source.
There are two ways to do.
The #1 is easy.
The #2 is also easy with command line.
In order to bring the forked project up to date, do the following:
git checkout master
git pull ssh://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git master
git pull origin master
The above steps are assume no conflict. If there is, you need to take care of it. A more detail instruction can be see from GitHub Help.