Using pathogen to install .vim plugins.

cd {git repository directory}
git submodule add {git path of plugin} {path in git repository where it will be installed}
git submodule init
git submodule update

For example to install supertab

cd ~/.dotfiles
git submodule add git://github.com/ervandew/supertab.git .vim/bundle/supertab
git submodule init
git submodule update