People never consider what is in their water when they use it to clean, this is very important with glass that clean water is used. If you're diluting your glass cleaner, consider using distilled water, this does not have all the minerals that can be present in the water from your tap, so it won't leave behind any streaky deposits on your bathroom mirror. In some cases you may be out of your usual glass cleaner or you're just looking for a cheaper option, vinegar can be a good product to use to clean windows and mirrors.
It doesn't take much soap to get rid of dirt, using too much will result in an overly dense cleaner that can leave a streaky residue on the glass. Paper towels leave not only streaks, but linty ones. Instead, go for a microfiber cloth, a squeegee, even a handful of newspaper can be used.