Spray talcum powder in the grooves of the hardwood floor. Try to have it penetrate in with a brush, and add some more from time to time. This should help lessen the "squeaky" noise.
Source: Common used knowledge in France
By Olga Proniewski from Miallet - France
This page contains the following solutions.
Locate which plank is squeaking and sprinkle some talcum powder in the cracks. Let it sit for a while so the talcum powder can work itself between the cracks.
