Currently, only packages that are registered in the General registry are mirrored. If you maintain your own registry of Julia packages that you would like mirrored, or if you have an unregistered package that you would like mirrored, please open an issue at https://github.com/UnofficialJuliaMirror/MirrorUpdater.jl/issues.
The mirror is automatically updated daily by means of a Travis cron job. I can also trigger it manually if need be.
The code for maintaining the mirror is available at https://github.com/UnofficialJuliaMirror/MirrorUpdater.jl. If you want to maintain your own mirror, simply follow the instructions in the README.