Unlike all the guys above, I always ride with the shield up - looking through it at speeds over 55. And, since I look through it, I take meticulous care of it. In addition to what two_wheeled said in #7, I use Novus windshield wax. They make several very fine polishes for windshields and depending on condition and age, you might need one of these. About a year ago I bought a Calsci windshield (marginally quieter and a bit taller than stock) and have been using the recommended mix of Rain-x windshield wiper fluid and regular windshield wiper fluid (50-50). Do not use any windex type or fluid that contains ammonia. Never ever use paper towels on your windshield (I've been told not to use these on glass windshields either) - I use cut squares of old flannel pj's that I keep for this purpose in my topbox along with the Novus and R-x/washer juice.
So, to sum it all up, Check out Novus products - you may need to polish out scratches (test on a lower corner). Keep that shield clean - get bugs off no later than the end of every day (I even do this at gas stops). Consider a new windshield if your old one is scratched badly. Plastic polishes are not cheap and may take a fair amount of work. Other products recommended on this site along with my edited comments from the guys who recommended them in ( ): Fastax (ok); Rejex (works well); Novus 1 (water beads); Ranshield 1 (water beads).
Other threads here for aftermarket shields - search for windshield - or PM me and I can send you several links.
A PS - I've been looking for Plastic Rain-x treatment. The current 'original' formula (a claim that I doubt based on conversations with Rain-x tech support) says not to use the stuff on plastic. There is supposed to be a separate product for use on plastic windshields.
BTW, under some circumstances of low sun angle, you will get blinding glare through glass or plastic. Such is life.