I was working on a pull request (in version 1.5.1+1707.g506df7b.dirty) on OSX (10.9.5) with Python 2.7 in a Conda environment. Using the current virtualenv FAQ (http://matplotlib.org/devdocs/faq/virtualenv_faq.html, specifically the frameworkpython script solution), I was not able to load matplotlib.

In the discussion of the pull request, @efiring suggested a procedure (#6358 (comment)) that did work after adding one final step. Inside the Conda enviroment you want to develop matplotlib in, the procedure is as follows (mostly copied from @efiring):

1. Use conda to make a test environment with the desired python version, and use conda to install matplotlib in it.
2. Use conda to uninstall matplotlib.
3. In your github clone of matplotlib, make a feature branch, and make the desired changes to it.
4. In the root of that clone directory run pip install ..
5. Install the Framework version of python with conda install python.app.
6. Open a new terminal and activate the conda environment (or run hash -r (bash) / rehash (zsh) in the old terminal to make sure your shell finds the conda version of pythonw instead of the OSX version).
7. Use pythonw instead of python from now on.