2 kinds of hook shots:

turn around half-hook: this is where u turn slightly facing ur defender, but so ur shoulder is still pointing to him. then, starting from a bent position, extend ur arm and release the ball with one hand, like u would in a floater or a power layup. go off two feet, really use ur legs and not that much arms in order to get good control. most post players do half-hooks: tim duncan has the best half hook in the nba right now (game 7s gonna be good btw)

practice doing the half-hook off jab fakes from the post, spin moves, pump fakes, up and unders, etc. its useful for both opst players and guards trying to take it inside.

kareem "sky-hook": this is a lot harder, and kareem abdul-jabbar is the only guy who has ever perfected it. u must have both shoulders parallel to the basket, so that u r looking at the basket from the side. then, going off one foot, "hook" ur arm in a swinging motion and relase the ball at its very peak.

heres a pic of kareem starting his signature hook shot:
http://www.basketballphoto.com/images/16636.jpg

if ur a post player, then mastering the kareem hook shot will give u an awesome scoring option and a cool nickname. but dont do this if ur a guard like me it takes a lot of commitment, almost as much as ur jumpshot, to make this effective.