Use this analogy... take a pencil and hold the sharpened end with one hand. With the other hand, use your finger underneath the pencil to create tension and then slide the pencil of that finger slowly and it snaps down....im sure you know what im talking about.
In the case of the slapshot for hockey you need to use the ice as the "tension" part. When taking your slapshot, hit the ice b4 the puck and create as much tension in the stick as possible b4 releasing the shot. thats how you get that good whip out of your stick.
As far as your balance issues go, I would try paying attention to where your bottom hand on your stick is. Try moving it down the stick a bit first. And remember to bend your knees.