Still have squeaks? Try these tips.
For one or two squeaking boards, toe-nail them into the subfloor (or floor joists) with finishing nails, then set and fill with wood filler. This can be almost invisible.
For a more extensive problem, screws are the best bet. Countersink them, and fill the holes with wood plugs. With wider-planked floors, you can make this an attractive part of the floor by using a different wood for plugs, and plugging every board, whether it needs screwing or not.
If you’re laying carpet or other material, you can always screw wherever necessary, but think about the future. If you ever want to remove the carpet, you may have ruined your existing floor.
Stairs
The surface methods used for floors can also help squeaking stairs, but if you can access the stairs from beneath, you can install a cleat beneath the squeaking tread. Have a friend stand on the tread while you fix it. From underneath, coat the edge of a 1×3 with glue, shove it into place below the tread, then screw it into the riser.
This method takes a little work, but gives you an invisible job from above: Remove any trim molding on the stairs, and drive small glue-coated wooden shims into any gaps (the location will depend on the stair construction). When the glue dries, cut off the projecting wedges, and cover all gaps with trim molding.