How do I silence or muffle creaky floorboards?
There are a number of products marketed for this purpose which basically drive headless screws through the floor to fix the boards in place so they can't move. Haven't tried them, but reportedly they work.

The other solution is to learn to ignore the creak. It's amazing what sounds we can learn to filter out once we accept that they're just part of the environment -- traffic noise in the city, screaming hordes of crickets in the country, my grandfather clock, ...