Interesting aspect, the throw-in is all important - up there with the goalkicking.
First, technique: almost everybody uses the two handed throw. If you think of your arm as a lever to move the ball, it is certainly not a very effective one because of the shoulder, elbow and wrist joint. They give the throw a lot of variables. With the two handed throw a lot of these are removed from the technique.
Next, upper body strength: a tight upper body provides the base for the movement of the arms. I saw a practise in the Wales Academy program where the hooker sat on a swiss ball to work on this stable platform. That hooker even managed to hit the target while kneeling on the swiss ball!
Then the target, a fixed target helps to focus on the throw. This is good practise but in the actual game there is no target. The trainer can use a target on a stick, raising the stick simulates the player getting lifted and the hooker timing the throw.
Last, the mental aspect: clear your mind to hit the target under pressure!