This will delete branches that have already been merged. Good for cleaning up local repos. Wont delete your current branch.
git branch --merged | grep -v "\*" | xargs -n 1 git branch -d
Saved here for my own reference but all credit to this guy: http://stevenharman.net/git-clean-delete-already-merged-branches
WARNING: This will only spare your current branch. Remember that you might lose master/staging/etc if you're not currently on that branch.