I like Simon's ideas about shared location and separate app (Django encourages us to use apps to share anything), but you also may share them on VCS level. SVN supports "external" checkout, in git you may add one repository to another.
\project1\templates\shared_temps
\project2\templates\shared_temps
"shared_temps" in both cases is just a repository (folder with .git subfolder). So, you commit in one folder, push, and pull in all other projects.