Well there is "cleaning", which should be done with a MF towel and a nice QD, like Poorboys, Clear Pearl, etc. Wipe in a linear direction only and never in circles.
I prefer to mist the plastic windows, get it fairly wet, and then use a blotting/wiping motion paying close attention to not grind any grit into the window. Don't use the same side for drying as you did wiping, in fact I usually use all four sides on two plastic windows. Flip and fold and use the other sides for the rest.
Then we have "polishing", which is done to remove fine scratches and blemishes. In a pinch you can use good ol plain white toothpaste, such as original Crest or Colgate. When I use to build scale models for fun and competition I would have a tube of Colgate in my model box to rub out jet canopies and car windshields.
As said Meguiars makes a nice plastic polish which was orginally formulated for furniture polish. I personally use the Novus Plastic Polish System. This is a 3 step system with #3 being the most aggressive of the 3 and should only be used to try and remove your deeper scratches and only apply it in the direction of the scratches. Apply it with a applicator sponge or a soft MF towel. Use a new towel and some QD to buff of the residue.
Step #2 is designed to then polish out fine to medium scratches and this is applied in a circular motion, small tight motions. I use a sponge applicator for this step and again use the QD and MF to buff off. I do this step 3 or 4 times.
Then apply the #1 which is very liquidy and mainly designed to polish an already clean surface. Works great on instrument cluster covers, wood trim on the interiors and plastic lenses on the exterior. Works great on sunglasses and I have used the system to also remove scratches on CD's!
Hope that helps....Oh, some folks also use Pledge on thier plastic windows and love it.
Anthony