The windshield washer fluid normally contains some deicer to help melt ice and snow on the windshield. It gets warm in the engine compartment and helps a lot. Spray periodically on the windshield when driving. Keep it filled. Buy the fluid which is rated the coldest temperature. It contains the most deicer (alcohol).
Defroster use is critical, as noted by the previous posters. Sometimes you actually have to pull over and scrape the windshield with a scraper.