Dismantle it to the bare frame, then give the frame a good cleaning. Get out all the dust, dirt, and rust out from inside the tubes. Maybe use a thin nylon brush with a very long, skinny handle. Next inspect the frame carefully for obvious cracks or holes in the tubes or welds. This is also a good time to double-check and be certain that your bottom bracket has a drain hole to let water out. Any spots that don't have paint on them should be touched up with good paint. At least a good automotive/metal primer if you can't get matching touch-up paint. Get a can of JP Wiegle's FrameSaver and apply a generous coat to the the INSIDE of the frame. Finish off by applying some car wax to the painted parts of the frame to keep it looking nice and protect it from salt on winter roads. Lastly, rebuild the bike by replacing the fork, seatpost, cranks, etc...