Anything that slows moisture transfer. Speed is the enemy of proper seasoning; you want the entire billet to lose moisture at roughly the same rate. You can use latex house paint, dip the ends in melted wax, whatever seals off the pores. Think of the wood as a bundle of rubber tubes, each of which is slightly expanded with the water inside. If the end grain isn't sealed, the ends lose their moisture faster and shrink faster than the middle; this can lead to cracking, both internal and external. If you seal the ends, the moisture works its way out through the sides of the billet, which takes longer, but dramatically reduces the risk of checking/cracking. Cutting the wood to rough dimension is a good idea, as thicker pieces generally develop more internal tension as they dry. Hickory is also a LOT easier to work while still green, so you'll be saving yourself a lot of aggravation down the road.