The absolute best I think you're going to be able to do is to isolate the source of the sound (or noise depending on your perspective) from the structure of the apartment. That will give you a little bit of "sound proofing" in that the sound will be a lot less structure borne, but it's going to be like taking an inch off 100 feet. Either move to a basement apartment, or buy a house. Besides that all I can recommend is to wear headphones if you're a guitarist, and use rubber practice heads if you're a drummer.