When my bearings are rusted I typically soak them in WD-40, work the rust out of the treads with my fingers until they spin smoother, then clean the WD-40 out with rubbing alcohol and blow them dry with compressed air (I use an air compressor but canned air works too). I only do this if I think I have a chance of saving them, if they're completely seized it might be worth just getting a new set of bearings.