The key to making moist meatballs every time is to add a small piece of ice into the center of each meatball. I know, it sounds funny, but it works. So right before you start to brown your meatballs insert a small piece of ice into the center of each meatball.
You may have to test a few first to determine your browning time, because you do not want the meatballs to end up raw in the middle. We are aiming for moister meatballs, not raw meatballs. It is a really simple tip for moister meatballs that is definitely worth trying.