I've found a lot of the products that work leave streaks if not rinsed well soon after applying, they also leave difficult to remove spots on the window. Most of the colored cleaners like Simple Green, Totally Awesome, and purple power, work pretty well, with the aforementioned things to be aware of. They can also remove any wax or polish.
A cleaner/wax like Mequiar's #50 works but is a little more work.
Keeping the roof clean minimizes it, but it's tough to keep the roof clean.
Depends how picky you are.