Mix a quarter cup of vinegar with a quart of cool water in a spray bottle, and use rolled up newspaper to wipe the windows. Don't worry -- the newsprint won't rub off on your windows, and it's cheaper than going through paper towels.
Inside the house, start at the top and work your way down the window so you don't have to deal with drip marks.
On the outside, work from left to right. This way if you have streaks, you will be able to tell which side they are on. Try not to clean the windows in full sunlight as it causes the cleaning solution to evaporate too quickly and leave streaks.
To remove any streaks, simply spray a bit more solution on the newspaper and wipe them away.