Most of us don't consider what's in the water we use to clean. In truth, it usually doesn't matter. But with glass, you see absolutely everything, so water content can make a difference -- especially if you have hard water.
If you're diluting your glass cleaner, consider using distilled water. It doesn't have all the minerals in it that can be present in the water from your tap, so it won't leave behind any streaky deposits on your bathroom mirror.
Of course, the cleaner you're diluting matters, too. Up next: It's the cheap route to streak-free.