If I can be bothered, I add any or all of the following to my meatballs: onions, garlic, parmesan cheese, lemon zest, oregano, chili flakes, egg, bread soaked in milk... I brown them about 5 minutes in a frying pan, then simmer in the tomato sauce about 30 minutes. They are perfectly nice with just mince, chopped onions and garlic though. I also usually combine equal amounts of pork, beef and veal mince and make a huge batch and freeze.
For burgers, if I have accidentally bought lean mince I'll either put a teaspoon of goose or duck fat (1 tsp per 500g mince) or I'll mould a pat of butter into the middle of the burger. Or replace the butter with blue cheese. When I mould them I make them into discs with a dent in the middle on each side. Because we like our burgers with bacon, we fry bacon first, then fry the burgers in the bacon fat for 3-5 minutes a side depending on how well done we want them. Best way to tell if they're done is to cut them in half (no prob if you're also going to cut the burger in half), or prod them - if they feel relatively firm and not raw any more you'll know they're done.
(Before you worry about the state of our arteries, we very very rarely have burgers because I figure it's not worth trying to make them healthy - would rather have one dripping juicy tasty burger a year than a dry and boring burger every week!)