Dry firing is as important as live fire and also free! It has the bonus of improving the trigger through wear as well. My 17 gen 5 has an awesome trigger after thousands of rounds and dry fire cycles. A slimline glock might never get that nice without modding, but it will get better.

A laser trainer can help identify imperceptible gun movement when you fire. A red dot (recommended if you can afford it) will do that too. Practice different finger placement and grip until you can take the gun through the trigger break with little to no movement. There's no hard and fast rule on little details like where your finger rests on the trigger and exactly where on your hand the backstrap rests - you have to figure it out based on your individual anatomy. Some people pull with the middle of the first pad, some with the first knuckle, and the only thing that matters there is if it works for you.

You do need to learn the basic thumbs forward combat grip, though. Most of the pressure should be coming from the support hand. If you grip hard with the dominant hand, you can tense up your bottom 3 fingers when you pull the trigger which will throw you off.

It's a lot more challenging than movies make it seem, but stick with it and you'll get it.