It's best if you have an outdoor rink within driving distance, and if you go early in the morning when there isn't many people there, take a couple pucks and shoot off the boards for an hour or two. Do that for the winter, and your shot will increase quite rapidly.
Rinse and Repeat. Basically take a ton of shots, getting stronger definitely helps, but just being strong doesn't guarantee a hard and useful shot. You just gotta keep shooting, eventually you'll find and consistently hit your comfort area.
Basically, all you want to do is throw the puck at the net, it's a simple concept. Before I tried making my shots look pretty. When really it's quite simple. Get the puck, throw on net.
For slappers, all I do it to fire it off hard and on net, I get it pretty low from the blue line but it's a really hard shot. My bro is more keen on making his shots go shelf, but the shots are really weak and are often off target.
Always keep in mind, Throw it on net. Imagine throwing a baseball, but instead of your arm throwing the ball, your stick is throwing the puck.
for forwards my advice is to practice your wrist shot as much as you possebly can. You can place small targets like pegs, water bottles, jerseys or foam targets in the corners of the net for accuracy. If your a forward you will rarely have enough time to fire a slap shot and if you do chances are you're in a low scoring situation. Most goals come from the slot area where accuracy is more important than any other factor.
for defencemans I agree with most posts above. The only thing I would add is to keep your shots low (below the knees) when a defenceman winds up, forwards will stand in front of the goalie to obviously create a screen. If your shooting for the top shelf your just going to hit the screen in front of the goalie. Also if the shots are low, forwards are able to deflect and tip the puck.
start by shooting an orange ball outside ur house on a wall if you can.
Once ur happy with ur shot with a ball, u can easily move onto the ice and just keep shooting pucks. with practice u learn how to improove ur shot.
I was always a fan of the plexi-glass. Get a surface that allows you to shoot pucks off of it, and that plus a lot of practice is a sure-fire way to improve the shot, no matter what kind you choose to practice (snap, slap, etc.)