pip is just a PyPI package like any other; you could use it to upgrade itself the same way you would upgrade any package:
pip install --upgrade pip
On Windows the recommended command is:
python -m pip install --upgrade pip
The more safe method is to run pip though a python module:
python -m pip install -U pip
On windows there seem to be a problem with binaries that try to replace themselves, this method works around that limitation.
In my case my pip version was broken so the update by itself would not work.
(inside virtualenv):easy_install -U pip