ride + brake

Just ride the bike slowly and apply brake gently for 50 - 100 metres most of the rust should  be gone. Now apply brake harder a couple of times (first check for traffic behind you) to make sure brake works fine, job done.

remove the pads so they don't get crap all over them, better yet replace them with Lyndalls after cleaning the disc. steel wool or a scotts bright pad then spray with brake cleaner and wipe off.
Best way to prevent it is ride it often

Shit hot, thanks so much guys.