Ideally, when it's to clean your car, you'll have access to proper car wash supplies. If none are handy, however, try Coca-Cola and a damp cloth.
As tips blog WonderHowTo points out, Coke can do a quick and dirty job tearing through thick layers of insects, grime and crap. Pour the soft drink on to a rag and wash your windscreen then rinse with a wet cloth or your regular windscreen wipers. Just be careful to avoid getting Coke on your paint job.
This isn't the best solution if your car is sitting in your drive way, but on a long journey, it could be useful. Check out WonderHowTo's piece for more road trip tips.
10 Hacks for Your Next Epic Road Trip [WonderHowTo]