I found this on https://coderwall.com/p/f4cnkg/a-better-way-to-setup-git-flow, and I want no illusions as to the source. I just know that I'll forget about it unless I put it on here.
$ git config --global alias.flow-setup '!git branch --track develop origin/develop 2>/dev/null ; git flow init -fd && git checkout master && git pull origin master && git checkout develop && git pull origin develop'
Now clone and setup tracking for any git-flow style repo by running the following.
$ git clone https://github.com/user/repo.git
$ cd repo/
$ git flow-setup