once your work on the forked project is done, then is the time to get everything back to the upstream.
$ git fetch upstream; git checkout master; git rebase upstream/master
if everything is ok this will go through hands-down. if there are merge conflicts you will be prompted, then
$ git rebase --continue; git rebase --skip; git mergetool
also every once in a while execute the following command to get rid of the .orig files that would appear after a merge
$ find . -name "*.orig" -print0 | xargs -0 rm
note: You can use kdiff3 as your merge tool.
More on Git