Squeaking floors are caused by gaps between either the joists and the subfloor or between the subfloor and the flooring itself (for wood floors). The easiest way to stop the squeaks is to get underneath the floor use screws to tighten up the attachments. If you have a squeaking wood floor, you can usually screw the loose board to the subfloor from below to get rid of the squeak. If you floors that are not wood bu still squeak, then you usually need to put screws through the joists and into the subfloor to secure.

That's all well and good. But what do you do when you don't have access to the underneath of the floor? You can a special type of screw that is designed to be put in from above: http://www.123itsdone.com/. These screw go in and then break off so that no screw head is visible. If you have carpeting you can put them directly into the carpet to tighten the subfloor to joists. Once the screw snaps, the entire screw is beneath the surface of the carpet and you can't even tell it's there. You can use these screws in hardwood floors. It does leave a hole, but the whole is much smaller than what you'd get if you had a screw head on the floor. Just fill the hole and you are done with your squeaky floor.