Frequent application of foot lotion (esp. right after a shower) and wearing socks all the time is probably your best bet. If it's really bad, you may have to gently pumice/"foot file" some places so lotion can actually do its job...
Straight shea butter, or a Bath and Body Works product (I can't think of it right now) work pretty well for me. (I am the queen of cracked heels. It's related to eczema as well as just dry skin for me, so if I'm stressed out, the cracks can get so deep they bleed.)