Things not to do to a burger:
Spank the monkey with the spatula
While I whole-heartedly agree with this statement in general, I have to admit that I sometimes crave a thin "smashed" burger. The key though is knowing how to do it properly so as to not press out all the juices.
First, I start with a 75/25 mix (this is my preferred ratio for burgers no matter what style). Form into a loosely packed 3-4 oz ball. Place in a screaming hot cast iron skillet and smash the burger to about 1/4" thickness. You must do this in the first minute of cooking while the meat is still cold and the fat has not started to render. Cook for 2-3 minutes or until a nice crust forms. Flip the burger (you will probably need to scrape it off of the skillet so be careful). Add the cheese, if using and only let cook for 1 minute longer, maximum.
This gives you a diner style burger with a nice crust and and little crispy edges, but one that is still relatively juicy.
While I, generally, prefer thicker, grilled burgers, if I find myself having to fry burgers this is usually how I do it.
As for mix-ins I am absolutely against adding anything to my meat before I patty them up (the exception being my chorizo burgers which is about 2/3's burger meat & 1/3 Mexican chorizo). I season liberally with Penzey's Turkish Seasoning (has become the household "seasoned salt" around here).
I have to admit that I am not a fan of cheese on my burgers. I don't know why as I love cheese and I love burgers, but I'd rather not combine the 2. Although, occasionally I do cheese on my burger. Yes, I know, weird.
As for toppings, while I occasionally like something a little different, I am mostly a classic burger kind of guy. Ketchup, mustard, thinly sliced raw onion and homemade dill pickles (and occasionally cowboy candy-sweet pickled jalapeno slices).