From DIYinfo.org
Loose and creaking floorboards may be caused by incorrect nailing, by the joists below them settling, or by warping and shrinkage. It is usually possible to cure a loose board simply by re-nailing or screwing it back in place.
But before you do this, check that the loose joint coincides with the center of the joist below, taking the board up if necessary. If it does not, widen the joist with a new bearer (See Pictures 7 -10 in Step By Step Pictorial View Of Repairing Wooden Floors)., or replace the whole board.
To nail floorboards, use 50 mm lost head nails or flooring nails. Position them next to, and about 12 mm away from, the existing nails. When you have finished, drive all the nail heads well below the surface of the board with a nail punch.
To secure floorboards with screws, use 40 mm countersunk steel screws. Drill pilot holes for them 12 mm from each existing nail, taking care that the holes go no deeper than the thickness of the board. When all the screws are in place, make sure that none of them protrudes above the surface.