Having looked into this a little bit myself, I second the Novus recommendation based on reputation.

As for what I have personally used, I have previously used Blue Magic's Plastic & Plexiglass Cleaner with decent success. It will work for minor scratch removal and light hazing. It's probably not the best product for the task, but for what it's worth I'm tossing in my review of it. It will do for minor fixes. It will start to look good before you should actually stop, so go the extra mile. Otherwise it will just temporarily mask the imperfections and they will reappear.

If you aren't experienced with a buffer (like me), it's probably best to steer well clear of 'em and do it by hand. Like others have said, it will most likely require multiple applications and LOTS of elbow grease, so don't be shy and work out some aggression.