Learn to anticipate the play -- Keep your eyes moving and watch the play develop. Don't prepare to make the hit too early because you will tend to make the hit even though it isn't the best option. If the player carrying the puck is skating near the boards, gradually move towards the boards if you are confident that he isn't going to turn the other way and blow past you. Then move into him. For an open ice hit, watch for him looking at the puck, and demolish him. You're better off keeping the player to the outside and allowing him to run out of territory.