I just smoosh up the meat (buy ground chuck or something fattier for burgers, like ground beef) with some salt and pepper and a little fresh onion. No need for an egg or bread crumbs, in my experience, but if you use onion, don't use too much or they won't want to stay together. You can buy a family-size pack of meat when it's on sale and make a bunch of patties to freeze (wrap 'em in wax paper so they don't stick together).