How to prevent meatballs from drying out when I substitute a lean meat?

You'll have different techniques for different recipes. The most important thing is not to overcook it ... but with ground meat and ground poultry in particulary, you want to make sure you've cooked it long enough to be safe.

For amalgameats, like meatloaf and meatballs, the common technique is a panade, breadcrumbs soaked in milk, and adding vegetables helps too (cooked spinich, onions, bell peppers, carrots, etc.)

For sausages, apples are a fairly common item in chicken sausages to add moisture. I don't know that the flavor combination would go as well with beef or pork sausages, but you generally want fattier cuts for sausage anyway.

For burgers, it's even trickier, as if you add a ton of stuff into the burger it becomes something that's more a meatloaf patty than a burger. You can make two thin patties, and put butter or cheese between them, and cook it as a 'stuffed burger', but if you're selecting leaner meats, I'm guessing you won't want to add more back in. Your better option is to cook the burgers in a low oven until they reach your desired internal temperature, then give them a quick sear to develop a crust; this will help minimize the amount of overcooked exterior.

Here's a great tip especially for turkey or chicken meatballs or burgers. For every pound of ground poultry, add 1 tablespoon of unflavored gelatin bloomed in 3 tablespoons chicken broth. Since the gelatin holds on to water, the meat will seem more juicy. Gelatin is made of collagen, that's what gives a good pot roast its unctuous feel.

To bloom gelatin, just sprinkle it over the liquid (lukewarm is fine). The gelatin will soak up the liquid in 5-10 minutes, then it is ready to add to the meat.

Of course you can do the same thing with leaner ground beef. Just use beef broth instead of chicken broth. A little Worcestershire would be great to replace some of either the chicken or beef broth.

I have a vegetable/fruit juicer and add the pulp to ground chicken meatballs and meatloaf because they tend to be dry without it. I use the pulp from juicing a mix of kale, beets and beet greens, apple, carrot, and ginger and it really works to moisten the chicken. The ginger adds a hint of Asian flavor, so omit if you don't want that. I am sure this would work for all types of meatballs, not just chicken. Another bonus is you get more vegetables and fiber in your meal! I use an egg to help bind and use no bread crumbs, so very low carb! (Just FYI, Tiger sauce makes a great dipping sauce for my Asian chicken meatballs)

For every pound of ground meat (even lean ground beef) I use 1 Tablespoon of ground flax seed-(flax seed meal available now in most grocery stores.

I'm not concerned about calories but I watch my husband's cholesterol. I select lean cuts & add some 'good' oil such as olive oil or flax. I find that grinding my own in a food processor makes it juicier & it is easier to incorporate additives. A courser grind is juicier

For very lean meats. I find 100/20 works. Or 100/20/10. 100% lean. So if 95% lean add 15lb ground pork suit. Make your patties & fry. 100/20/10 is good. 100% lean 20% fat suit added, 10% spice, bread crumbs or other added. fine ground. In a mix.Turkey is not native were I live. This does work on peacock a native bird. Also ground lean meat can be spiced. fat added if need. Stuffed in large pasta shells. Steamed in a closed pot or steamer on a rack. In oven or steamer. Till cooked. Refrigerated in a pan on rack with a little water in bottom or froze. Do not over steam. Then latter microwaved. Or added to sauce. for use. Steam will add moisture to the meat. Such is good set out with rice, &, steamed vegetables. A spicy soysause mix goes well here for a dip. % can be varied for your taste. The trick seems to be add fat or oil to the meat. Before cooking.