Take it apart, apply some steel wool on the rusted areas to remove it, oil everything well, re-assemble, and continually keep it well-maintained. That's about all you can do. And keep your other magazines maintained to prevent rust from occurring.
+1... Just don't get too carried away with the oil especially if you keep your mags loaded.
I take an rag made from bedsheet (doesn't seem to leave lint behind) and and get it damp with oil. Just enough to leave a slight oily feeling on your hands when you use it, and a wood dowel and run it back and forth in the mag till I'm sure I got everything covered.