Put down a rug or carpet. I know, its not really a solution, but it works. My friends live on the bottom floor and when the upstairs neighbors remodeled and removed the carpet it got really noisy. We could even hear them walking. There isn't a whole lot of options when you share floors and ceilings, but rugs tend to cushion the noise from above.
You might ask in what room they are hearing the squeak. It could even be water pipes making noises.