Compound on a buffing wheel, same as you'd use for gelcoat. If the haze is heavy, start with a cutting grade, then go to a polishing grade.

If you're not familiar with compounding, and you're buying something specific for this task I recommend Novus products (disclaimer - no relationship). Start with the fine scratch remover http://www.novuspolish.com/fine_scratch_remover.html I recommend you use a very clean wheel that's made for fine polishing -- there are different grades of wheel for cutting than polishing. An auto supply store will have a variety, and made sure you use the right kind. A wheel for cutting will be much more aggressive against the soft plastic, so don't use that.

Then finish and maintain with Plexus or another plastic polish that contains UV inhibitors.

Alternatively, any product made to polish out the haze to restore plastic headlights should do the job.