If necessary, dust off the window and sill with a clean paintbrush. Excess dust and water can cause mud.
Use a professional-type squeegee available for about $20 at a janitorial supply store. Forget the cheap brands you find at the grocery store. They are not as effective, and you have to replace the whole thing once you get a nick in the blade.
Don't clean windows while they are in direct sunlight. Your cleaning solution will dry too fast.
Dip a 100% cotton cleaning cloth in your solution. Wring out the excess and then wipe the window to loosen dirt.
Grab your squeegee. Start each squeegee stroke in a dry spot. Wipe a strip with a cleaning cloth to get started.
Squeegee in a pattern from top to bottom, or side to side. If you clean the outside and the inside, Work top to bottom on the inside and side to side on the outside. By doing this, you'll be able to identify which side any streaks left behind are on.
Keep the squeegee blade dry by wiping it with a cleaning cloth after each stroke.
Replace the blade when necessary. Even the smallest nick can cause streaking.
Don't have a squeegee? Use newspaper for drying freshly washed windows. It's cheaper and leaves no lint behind.
More Window Washing Hints
Wash windows on a cloudy, but not rainy day. This is the best time to vacuum the frames and sills. Cool, clear water is the choice of most professional window washers. If windows are very dirty you can add 2 to 3 tablespoons of vinegar per gallon of water. For drying windows, a wad of crumpled newspaper works just as well as expensive paper towels. Wear rubber gloves to keep your hands free of ink.

Cleaning Window Sills
To remove spots rub the surface with rubbing alcohol.