To hook the ball out of trouble---for example, around a tree---exaggerate everything. Close your stance more, turn your hands more to the right and make an effort to turn the club face over even more through impact.
Switch to a flatter swing plane. It's difficult to hook the ball if you possess a steep, upright swing. To promote the hook, think of swinging as if you were hitting a baseball.
If you're hitting the ball straight right, you probably have an inside-out swing path, but you are failing to rotate your hands and arms to close the clubface. Try hitting the ball with the toe of the club. This can't be done, but it will force you to close the clubface.
Smaller grips enable the hands to turn faster through the shot, which produces a hook. More flexible shafts also promote a hook.
Use a tighter grip than normal when you want the ball to hook. If a normal shot calls for a grip of "5" on a scale of one to 10, you should grip the club at a "6" or "7" for a hook shot.
Remember, some hooks can be too wild. A hard, low hook that dives to the left is called a "duck hook" and tends to put a player in much worse trouble than a weak slice to the right.
Don't confuse a hook with a pull. A hook starts right and curves left. A pull starts left and goes farther left.
If you are a young, strong player who likes to swing hard, the hook may not be for you. Jack Nicklaus always had plenty of distance, so he favored a high, controlled faded shot---the opposite of a hook. Nevertheless, you'll still want to be able to hook the ball when you need to get out of trouble.