I know this is an older post, but what I used to do on my '94 Ford Tempo was soak a cotton buffing wheel in water then put a light coat of polishing compound on it. I'd chuck that in my drill driver, and with light pressure, go to town. I'd wipe off the lense every now and then to check on my progress. When it was as good as it was going to get, a nice coat of UV protection wax kept them looking good for a few months or so. Then I would do this all over when I saw them yellowing again. It took a little bit of time, but was way cheaper than buying new headlight lenses.
L8R,
Matt