There's a lot of great suggestions in the replies. I haven't tried them (except chrome polish, which worked okay). I'd try the soaking solutions if your parts includes threads, areas that are difficult to get to, or parts with irregular shapes.
That said, I have had great success using electrolsysis to get rust off parts. There are plenty of tutorials on YouTube. There's a bit of setup involved, but its results are spectacular.
$.02 to ya