Among the bottles in the cleaner aisle, you'll find glass cleaners promising streak-free results -- but what they don't tell you is that those results often come with a catch: a lot of elbow grease.
You don't need to buy a special glass cleaner, although the best is a non-abrasive window cleaning solution -- you probably already have a cleanser in your apartment strong yet gentle enough for the job. Soapy window cleaners -- a few drops of liquid dish soap in a bucket of warm water, for example -- will work on cutting through dirt and grease and will also work well with your indoor squeegee technique (and it's also safe to use on tempered glass). When you use a soapy or sudsy detergent remember: The less you use, the better. Use too much and these products can cause a soapy residue to build up on your windows, causing streaks and dull, dingy glass.
A one-to-one ratio of vinegar to water also makes glass sparkle, and can be used in place of a soapy cleanser (although it probably won't smell as good).