When washing both sides of a window, it is best to whip horizontally on one side and then only vertically on the other.  This way, when you see a streak, you will know which side it is on.

For cleaning windows, I use warm water with just a little bit of Dawn dish soap.  Seems to help keep the streaks down.

Another solution you might try, add 1/2 to 1 cup of corn starch to a quart of water.

A nice flat squeegee can help clean then without streaks.  Just be sure to wipe of the squeegee blade between swipes and word from the top down going from side to side.  If you are really talented, you can make a sweeping curve at the end of each horizontal swipe and not have to wipe off the squeegee between swipes.

If you subscribe to the newspaper, you can use that to wash your windows with and save money.

One last thing, it is best to wash the windows when it is warm.  Too cool, it will take forever to get them dry and if it is a hot day, they will dry and leave streaks and spots before you have a chance to dry them off.