What are some tips for cleaning windows without leaving streaks?

Make sure you don't stop in the middle as you're moving your squeegee, for one. Window cleaning (particularly working with a squeegee or WFP) takes some practice for sure.

Here in Las Vegas, it is often requested to clean windows with no chemicals. It is simple to do this streak-free if the humidity is low. Saturate a clean rag in warm water. Then squeeze almost all of the water out so the entire rag is the same slight dampness. Then wipe in a motion back and forth or circular. If done correctly it will clean the window and you will see any wetness dry on the glass almost immediately.