I read in a magazine once that a hand and foot model reported that she always used vaseline then covered her feet with saran wrap. This works the absolute best for me if I soak first, use a callous remover (I purchase it at Sally's beauty supply), then use a foot file to remove the dry skin before coating my feet in vaseline, wrapping in saran wrap, then a pair of socks. I do this before bed and leave it on all night. It works complete wonders! If I keep up with using a moisturizer every night then they stay soft for a while. I wear flip flops pretty much year round so it takes a toll on my heels.
I also recently found cocoa butter vaseline that has become my new favorite! I smells heavenly and it is a bit creamier than normal vaseline.