The two most common mistakes players make in failing to create great spin are 1) a lack of smooth acceleration, and 2) grazing contact, which are the two pillars of creating spin, whether it be serving, pushing, chopping, or slow looping. (For faster looping, you sink the ball more into the sponge, but the smooth acceleration part holds for all looping.) 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 looping, players below the advanced level often try to get extra spin by "muscling" the ball, meaning they try to use nearly 100% of their muscle power as they go for maximum racket velocity. The actual result is they only use a few of their upper-body muscles (often mostly arm), and a loss of control. When they learn to slow down and use all their muscles (at perhaps 70%) and smoothly accelerate into the ball, that's when they get the acceleration needed for powerful loops with both spin and speed. That's why the most powerful loopers often make it look effortless.
When serving and pushing, beginning and intermediate players often use a short stroke 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 more spin if you smoothly accelerate into the ball, with the rubber literally slinging the ball out as it accelerates through contact. (When looping with both speed and spin, where you sink the ball more into the sponge, it should feel like you're almost holding the ball on your racket as it carries it through the shot, with an even greater slingshot effect.)
But on slow spin shots (serving, pushing, chopping, slow loops), you only get this tremendous spin if you graze the ball - the second problem many players have. Too often players sink the ball too much into the sponge instead of the fine grazing motion needed. To learn to graze the ball, just toss one up and graze it with your racket, making it spin. Generally do this with a pendulum serve motion, but contact the ball on the left side of the ball (for a righty), with the racket going mostly up and slightly left, so that the ball goes straight up. Catch it and repeat. It's important to spin the ball so it goes straight up, both so you can catch it and repeat, and so you can develop ball control. (If you can't control the direction the ball goes when you graze it with this exercise, how can you do it when actually serving?)
I recommend beginning players get a multi-colored ball or put markings on one so they can practice grazing and see the spin they are creating. This gives feedback on whether you are really spinning the ball or not and how much.
For more advanced players, I recommend they also do the ball spinning drill I wrote about above. It's a great way to really develop those grazing skills so you can both spin the heck out of the ball and control it. Advanced players should also experiment with smooth acceleration and grazing on their spin shots, and see how much they can make the ball spin.
When you can put great spin on the ball with your serve, apply the same principles to pushing and slow looping. (Also chopping if that is in your arsenal.) Don't be afraid to throw in some slow, spinny loops, even if you normally loop pretty hard. Slow, spinny loops are extremely effective at the beginning/intermediate level, but many forget or never realize how effective they are even at the advanced level if not overused. They not only are effective on their own as the opponent struggles to adjust to the slower speed and higher spin, but the contrast makes your other loops more effective.