Try Rocco's Mama meatballs they are moist, it includes chicken stock. It calls for plain breadcrumbs. I make fresh ones by buzzing white bread (pepperridge farms, not balloon bread) in the blender.
http://www.roccodispirito.com/recipes/mama_smeatballs

usuba dashi is right, be gentle with your meat balls. :)

Also, the more you mix it the more you bring out the protein in the meat that can over bind the meatballs. But the panade (fresh bread crumbs & milk) idea is the way to go.

I like the milk/bread idea. I often use milk and oatmeal.. about 1/2 cup quick oats and 3/4 cup milk. The oatmeal mimics the texture of the ground beef, and you never know it's there.

Mix in 4 slices of water soaked white bread (discard the ends) to a pound of ground beef. Add fresh ingredients: 4 small chopped cloves, fresh ground garlic, chopped parsley, and 8 teaspoons of pecorino romano grated cheese. Mix beef, ingredients with 2 eggs and form meat balls. Brown meatballs in olive oil then add to your gravy (sauce). Cook gravy for 2 hours.

IIf you use whole wheat bread, they will not be tender. White bread is the key.

Thank You, i was using bread crumbs from the box.

I find that mixing in some milk-soaked bread makes my meatballs moist. Soak your bread, then squeeze out the excess milk (discard milk) and mix in the bread with the meat mixture.