When you’re taking a wrist shot the puck should rest on the heel of your blade and roll from heel to toe as you're releasing your shot. Your weight should be resting on your back foot as you're loading and transferred to your front foot as your shot is released. Following through on your shot by pointing to your target as you release your shot will greatly improve your accuracy.
The best way to improve your wrist shot is to simply practice. Don't just shoot aimlessly though – make sure that you're shooting to score every time. Pick corners, aim for the inner edges of the posts, practice shooting from tough angles on the sides of the net and release the puck as quickly as possible. Have someone pass to you so that you're forced to settle the puck quickly before releasing your shot. Accuracy and shot power are very important, but a quick release will catch the goalie by surprise so make sure you spend time practicing shot quickness.