We get our ground beef from someone we know who has a dairy farm, and the meat is rather lean - because they're dairy cows? I don't know. Anyway, it make for dry-ish meatballs. I once tried adding olive oil but it made no difference. Then one day I thought of adding some oats, my reasoning being that oatmeal is slimy and that might be a good thing. Well, it worked!
I put in about two tablespoons per cup, with everything else the same. The only time it doesn't work is when I forget to add it.