The best way to crack acorns is with a “Texan Nut Sheller”. Even after thirteen years it is still razor sharp. It works especially well on dry acorns. Be careful it is very sharp, NO KIDS and WATCH YOUR FINGERS!
Suellen Ocean is the author of Acorns and Eat’em, a how-to vegetarian cookbook and field guide for eating acorns. Find it here: http://www.amazon.com/Acorns-Eatem-How–Vegetarian-Cookbook/dp/1491288973