strengthen the middle of your body abs, "midline muscles," obliques etc
^^^ what they said ^^^
Develop your "core" which is your torso, abs, obliques hips, ets. Power comes from your core, not your arms, as it's generated by you properly transfering the weight from your thighs as you wind up. This is why you see those skinny, scrawny 135 lb. kids shooting so hard.
Also don't listen to those people who say to just develop your biceps. That's only like 25% of it.
The New York Times had a write up of the mechanics of power in slap shots and interviewed Sheldon Souray. Say what you want about his defensive play, but he has one of the most powerful shots in the league. His key to success?
Concentrating on his core as he perfectly transfers and balances his weight from core/hips through his windup to his actual shot. He didn't mention anything about his arms.
Of course he works out with personal trainers etc who have him concentrate on working his core by using pilates, those big plastic balls, elastic threads etc.
It's nice to be rich and have a personal tranier telling you what to do and what to eat, eh?