Do you know how to remove wiper marks from a windshield? There are scuff marks on there that don't seem to budge no matter how much elbow grease I put into it.
I have successfully used elbow grease before alongside a squeegee or a microfiber towel. Automotive glass cleaners give good results despite being a bit costly. On the other hand, some people opt to use a home-made water and vinegar mixture or rub pure ammonia on the windshield.