This is a tough concept for a lot of people. I once heard someone say “Maximum meat on the gun”. Maybe not very eloquent but an excellent point. Simply put you want to maximize the contact with the surface area of the grip.
Talking to the Semi-Auto shooters here: You start by wrapping your strong hand around the backstrap (the back of the grip) so that it fits snuggly into the webbing between your thumb and index finger. The goal is to be as high as you can. Stretch your index finger along the frame, and then curl your fingers around the front strap under the trigger guard. Then take your weak hand and get as much of the heel of your hand on the grip panel as you can (again maximum contact with the pistol) stretching your thumb forward, (and parallel under your strong hand thumb) aligning with the strong hand thumb, and curl your fingers over your other hand fingers, under the trigger guard. Your knuckles on both hands should line up, and there shouldn’t be any air space between any of your fingers and the others. How does it feel? Can you support the weight of the gun comfortably? Can you see daylight between the heels of your hands? If you can, you need to bring the weak, or support hand in a little tighter. Are you high on the backstrap without blocking the slide? Some guns have a tang, beaver tail or protrusion that prevents you from being too high, like the 1911, but not all.
When you take your shot, does the slide abrade your hand? Does your grip come loose and you have to readjust? Neither of these is desirable. You don’t want to cut yourself, it HURTS! You don’t want to readjust your grip constantly, it means you are breaking your grip, wasting precious seconds, and then still need to reacquire your site picture.
Recently, my instructor told me I needed to be higher on the backstrap. Looking at my grip, I probably would have said the same thing to a new student. When I tried to inch up (fraction of an inch actually) I found the recoil was more painful, I was adjusting my grip every couple rounds, and I started anticipating the recoil. How did I know? My shots were centered and low. I kept trying to find the sweet spot for me, and when I did I was once again drilling the dot. However, it was a little lower than I would advise. So…now what? I had good control, good grouping, but my grip looked a little off. Mmmm. Ok, admittedly, we all have a sweet spot, but they should be pretty close. What does this mean for me? I needed to practice with a higher grip until I could control it and then have my instructor re-evaluate.
What is the take away for you? A couple things, 1., be aware of your grip, and make sure you work with someone who knows to ensure you have it correct. This is important because it is a lot harder to break a bad habit than to form a good one. 2. If you are consistently shooting low center, you are anticipating the recoil or jerking the trigger, assuming you had the correct sight picture, and there are ways to overcome that, but it takes practice. One way is to have someone else load your magazine and insert a dummy round. (ball and dummy technique). You won’t know when it is coming up and you will see yourself drive the barrel down. That will help you realize that you are doing it and give you some feedback to break yourself. Another way is to safely practice dry fire with an empty cartridge or coin on top of your front sight or your slide. Does it fall off or stay put? I'm not a huge fan of dry fire, it can build in habits since there is not recoil, and you must manually rack between trigger pulls. But some swear by it.
If you have smaller hands, you may have a bigger challenge finding the perfect fit. I had a student recently, who when we compared hand sizes, we were very close, except the index finger, mine was nearly an entire knuckle longer. I can handle a Glock with ease, she couldn't reach the trigger and maintain a comfortable grip. It is critical, when you are shopping for a gun, that you check the fit, You should be able to keep a proper grip AND reach everything you need to, i.e., trigger, mag release, safety if you have one...
For a while the only option for someone with smaller hands was a single stack, or a slim revolver. However, manufacturers are starting to respond. Some find that the Springfield HD fits better. The Smith and Wesson M&P has 3 sizes of backstrap that comes with the gun. Gen 4 Glocks have the ability to make the backstrap larger. Just like shoes, one size does not fit all but your perfect fit it out there.