When i make mine i just use ground beef, one egg (to help them stick together), seasoned bread crumbs, garlic powder, onion powder, mustard powder, and a bit of parmsean cheese. It just depends on what flavors you like. My father makes his with ground beef, finely chopped onions and bell peppers, and mustard.
i like burgers with avocados. its fresh and has lots of nutrients from the avocado. also the avocado make the burger have smooth tecture.
All I know is don't squish them down when u flip them, it will make them dry, so I have been told
Here's an interesting idea: http://www.funadvice.com/howto/turtle_burgers