It was either on America's Test Kitchen or Good Eats (neither of which I get here in Afghanistan
) that they showed something that sounds nasty, but makes a great burger: milk and bread. I kid you not. Especially if you have folks that like a (gasp) well done burger, milk and bread help keep it super moist. I forgot the ratio of milk and bread to beef, but last time I did this, even my friends who only like well done were flabbergasted.
On seasoning, I'm a plain salt/pepper guy, but occasionally like to add make a greek-style burger where you add some finely minced onion, rosemary, and feta cheese. Salt a pepper to taste. Can't go wrong!