As soon as the skin starts to lift and rub, but before you have blisters you should tape your hands loosely with Zinc Oxide tape, from the chemist, this is really smooth and stops them blistering so your callouses develop as fast as possible. Once you have them the only way to do it is to pop them with the smallest pin you can find, sterilised in a flame and then cooled in ice, and then open them enough to super glue the skin down and tape as above, however try to leave your hands as untaped as much as possible so they can breathe or they end up smelling minging! Once you have callouses then you're cooking on gas because you should be able to ride as much as you want and not get blisters, but if you do tape them as above so that you don;t loose the thickened skin. You may want to try a firmer grip, it sounds counter-intuitive but it'll rub your skin less because you'll slide round rather than having your skin move. Also try fatter or thiner grips, if they don't suit your hand size then they will rub more and feel less comfy and give you less control.