Meguier's also sells a few products you can use. One system is a 2-part system that works great if the window/plastic is really stained or discolored. You put on cleaner #11 (or something like that) then follow by #12 polish.
Regardless of what system you use, like even that 303 stuff, also get the 1-part spray combo cleaner/polish Meguiers sells. It is handy to keep them clean in an instant. My experience with them is to keep on top of them so that dirt and stains don't have a chance to build up.