A big problem is the evaporation rate. If you're cleaning your windows on a warm sunny day, the liquid part of the solution evaporates quickly and leaves behind a residue that appears as streaks. On the other hand, if you don't entirely remove the liquid you can also be left with a streaky window.

Window cleaners usually use basic dish soap to clean windows effectively. This includes Dawn, Palmolive, and other standard products. Some pros also rely on specialized cleaning liquids.

Combine two parts water with one part white vinegar in a spray bottle and you'll be all set with a homemade window cleaner. You can also combine warm water with a few drops of dishwashing soap if you prefer. Invest in a glass spray bottle to mix up your formula.