Smooth Acceleration + Grazing Contact = Great Spin
This came up last night in the Beginning/Intermediate Class I teach on Monday nights. The two most common mistakes players make in failing to create great spin are these two, which are the pillars of creating spin, especially when serving, pushing, and chopping. It's true for looping as well, but only for slow, spinny loops. When you loop faster, you sink the ball more into the sponge. (I'm mostly writing for players using inverted sponge, but the same principles apply to most pips-out surfaces as well, as long as they have some grippiness.)
When serving and pushing, beginning and intermediate players often use a short stroke (to help with control) and sort of jab at the ball. They are thinking that the velocity they get with this jabbing will create great spin. Actually, it just leads to a loss of control as you can't control the racket this way. Plus, for physics reasons I won't get into (partially because I'm not a physicist), you get far more spin if you smoothly accelerate into the ball, and almost hold the ball on your racket as it carries it through the shot. This literally slings the ball out with tremendous spin.