There are a number of different things that may be causing your floor to squeak. However, the two most common are a warped or sagging beam and a warped floorboard.
If you have a warped beam in your flooring structureis will cause a dip to develop. First you will need to locate the beam that is sagging. This can easily be done by having one person walk across the floor while the other person is listening underneath. You will notice a gap between the joint and the subflooring at the location of the squeak.
Next, find a good piece of hardwood that is not damaged or warped. Place a board against the joist and flush against the subflorring. Now, simply nail the board to the joist. Problem solved.
The second problem, a warped floorboard, can corrected just as simply. Once again, you first need to determine where the squeaky board is located. Look for boards with the edges curling upward. These may be easier to find with a level or something similar that should lay flat across the floor’s surface.
Once you have located the board that is the culprit, you simply need to drive nails on each side where the board is curling. Keep in mind, you may need a couple for each side before you get the board to lay just right.
Finally, for older floors with several squeaks, consider taking a weekend to put bridges in between your floor joists for added support.