It's the most important part of the burger. Sure, some of your guests may top theirs with all manner of nonsense that may overpower the flavor of the beef, but you want to start with a good base. Chuck that's freshly ground from a butcher you trust (or that's ground at home) is a good place to start. If you're feeling up to it, you can experiment with mixtures that use different cuts, but you can't go wrong with chuck—it has the requisite fat content for a juicy burger. Go no leaner than 80 percent.
Mixing the Beef
Mix the meat as little as possible, whether you prefer the simple addition of salt and pepper or more exotic mix-ins. Overmixing leads to toughness. Toughness leads to anger. Anger leads to burgers you'll hate. Just form some loose patties quickly and without much fuss. They may not be perfectly round or what not, but they'll taste better. And who cares what it looks like once it's on a bun?
Until a few years ago, I had a problem. No matter how even the thickness of my patties, the center of the burger would swell up on cooking. But I learned a trick from coworkers at my previous employer—make a depression in the center of your patties. With thicker edges and a thinner middle, the swelling will eventually even things out.
Flip Once, No Pressing!
That spatula in your hand? It's used for flipping the burgers and used for flipping them once. You know what it's not used for? Pressing down on the patties while they grill. Don't do that! It releases all the precious moisture into the flame.
Flare-ups are a part of grilling. For most, simply close the lid of the grill; the reduction in oxygen should be all that's needed to quell the flames.