I suffered from this last year, bugger it hurts when you get cracks on your thumbs (pushing on the tamper is hell). Few ideas moisturise after washing your hands, the soap dries them out. Also there is a product called "tough hands" (or something like it) I got from the pharmacy, expensive (ie $15) but it is good stuff. Use it regularly and you will see a difference. Mine took a while to toughen up but now they are fine.
I also remember a bloke who used to lay concrete tell me that he used to soak his hands in baby oil for about 5 mins every night when he got home and that prevented the same thing.