The most important thing is not to use anything abrasive to clean the film. I would also avoid any glass cleaners that contain ammonia. Back in the days of dyed polyester films, the ammonia would cause them to turn purple over time.
With the new metallic & ceramic films, I honestly don't know if ammonia is still a concern, but there are so many great window cleaners out there that don't use ammonia that it's not worth taking the risk. I like Prima's "Clarity", but if you're looking for something that you can find in an auto parts store, Stoner's "Invisible Glass" is good stuff, too.
Use a thin microfiber cloth for cleaning the windows, rather than paper towels. Paper towels have a glue binder that holds the fibers together, and that glue can leave streaks/smears on window glass.
Lastly, when you're cleaning the outside of the windows, wipe side-to-side, and when you're cleaning the inside of the windows, wipe up-and-down. That way, if you see streaking, you'll know immediately if it's on the inside or the outside!
