The two most important things are to breathe deeply and correctly, so your muscles have enough oxygen, and to stay totally relaxed and fluid.
Your angle of attack depends on a lot of things. I pull the strings with the pads of my index and middle fingers, and my middle finger is about 1/2" longer than my index. I try to angle my hand so my fingertips line up on the string. Other folks use their middle/ring fingers.
The other important thing to remember is that slapping looks violent and physical, but it's actually pretty gentle. Hooking your fingers under the string is a waste of energy and slows you down. Stroke your finger parallel to the floor across the string until the ball on the pad of your finger catches the string, let it catch and pull just a little, then release. The dribbling-a-basketball analogy is exactly what I do. I'm no Joe Fick but that technique works really well for me.