I got most of the following information from Total Female Hockey.
The further back you place the puck, the lower the shot. Put the puck near your front foot for a high shot. Put the puck near your back foot for a low hot.
Place the puck as close to your body as you can without impeding arm movement. Don't make the mistake of putting the puck too far away from your body.
Use proper grip. Slap shot grip is different than stickhandling grip. Turn your wrists away from you, such that the blade closes a little. The bottom hand should rotate so that your elbow goes from slightly bent to being locked straight. Similarly, the top hand should rotate so that your elbow is locked straight.
Keep your bottom arm straight and locked down
Get low and stay low through the shot and follow through. Lead with the bottom hand.
You must turn the blade over and closed during the follow-through to have power and accuracy in your shot.
I've heard recommendations that you should hit the ice 6-10" behind the puck, rather than the common refrain of 1-2". Supposedly the pros use the 6-10" distance whereas beginners start with 1-2". The reason for using a longer distance is that it gives the stick more time to flex and makes your shot more powerful.
There are lots more slapshot tips at Wild About Hockey. When I start to really practice my slapshot, I'll write up something more complete.