more fat = moister (and better IMO).
Our basic recipe is (I'll double check when I'm downstairs and edit if needed on the proportions):
1 lb ground beef
1 egg
1 slice fresh bread (we've used wheat or Italian), soaked in water OR between 1/2 and 1 cup breadcrumbs with a spash of water (depending on who is making them, and our mood and how the meat mixture feels...meatballs IMO are a lot about feel)
spices to taste (we use salt, pepper, garlic powder, onion powder, oregano, and basil)
we make them just a little larger than a ping pong ball (using a small ice cream scoop to keep them uniform), and then bake at 350 for about 15-20 min, and they've not been dry. We don't cook them in the sauce, because we don't want the sauce having meat juice/flavor...