Really. I mean it. And all the other interior glass surfaces, too, while you are at it.
You're driving at night, and everything seems just a bit fuzzy. You think little of it, attributing it to being tired, or maybe, perhaps, needing new glasses. The next day, when driving, you notice that everything is fuzzy... and this is during daylight hours. You start pondering whether you need new glasses or not.
Perhaps you do need new glasses. But first, clean off the inside of your windshield! Use glass cleaner or some other sort of cleaner that won't leave a residue. Heck, even just use your shirt sleeve.
You will notice that everything suddenly looks a lot clearer, sharper, and the contrast is better. That's because the windshield was dirty! It does not look dirty so easily as the rest of the car, but, like all surfaces, it attracts dirt. In addition, the heat of the sun causes the plastic of the dashboard to decay, which builds up on the windshield.
This is more noticable on cars that have more plastic, and less on cars that have more leather upholstery. Either way, clean your windshield and other interior glass surfaces in your car once every month or so. You will find it much easier to focus on the road and on driving.
Yes, I always forget to do this... hopefully noding it will help me remember.
Withnail suggests: scrunched-up newspaper cleans really well - something to do with the ink in the newsprint.