I just got a new set of headlights, and was wondering if there was anything i could do to prevent them from ever going yellow again? Maybe a clear window tint? or using plastic polish weekly on them?

Headlight protectors will be your best bet. Just the annoying ones that go over the headlights. They only go yellow because the heat off the bulbs softens the polycarbonate lenses. As you are driving, dirt/dust particles stick to the lenses, causing the yellowness that you see. I had headlight protectors on my headlights, and they only went yellow right at the edges (as in the parts that are still exposed when the headlight protectors are on the car).
I am not too sure if the stick on ones will do the trick, but the genuine ones (which is what I had) worked.

Another thing would be to go the HID route. It will not stop them from going yellow, but it will delay the process. Most HID kits on the market are only 35W, and the lens does not get as hot while you are driving.

To permanantly fix the issue, get rid of the standard lenses. I am not sure if you can get them anymore, but there was a mob that were doing glass lenses for the EF/EL fairmont/lane. I last heard of this a few years ago, so I am not sure if they are still doing them. The other options are to go the glass lenses (either from a twinlight or an XR front). By doing this, you will also gain projectors for your low beam, giving you better light output (not that it is hard to get better than the standard lights).

thanks for the reply mate, im running HID's so hopefully it doesn't yellow while i still have the car. Might also just get the stick on film, as i hate the look of proper plastic headlight protectors..

the standard original headlights have a uv resistant coating on them, the lenses will start to yellow when then coating wears off. some of the first aftermarket lights made did not have this anti uv coat.
it usually starts on the top edges where the actual light has been polished by over zealous painters,or gets bugs cleaned off with scourers etc. eg when the bonnet gets resprayed the painter will get polish on the lenses and just buffs it off. it takes some of the uv coating off and will then start to yellow.
i have genuine ford replacement headlights on my el futura and they are over 5 years old and still look as new.
the stick on headlight film also is uv resistant so that will help the clear plastic lense last