Don't use lean beef. Fat is your friend for juicy burgers.
Don't add too much to 'em; garlic, parsley, dijon, salt "n pepper, that's it.
If adding onions to the meat, caramelize them, raw onions are always better as a slice on top with fresh tomato and lettuce.
Don't add fillers (oatmeal, bread-crumbs, etc) it turns your burgers into meat-loaf.
Don't mix it too much, they get tough.
When making patties, make the centre thinner than the edges. As the burgers contract with the heat they won't turn into rugby balls.
If you want to get fancy, chop up some smokey bacon into little bits and fry it crisp. Cool it with some of the fat. Mix the bacon and some of the fat with grated cheddar cheese and make a 2" diameter log. Freeze it. Slice thin. Build your patties around these slices. As the burgers come off the grill they will have molten bacon & cheese cores. Yum!

PitBoss, I agree with you about not adding fillers such as breadcrumbs to the minced beef when you make beefburgers, however, I only ever use lean meat in the burgers that I make,I add seasoning and work the meat with my hands for a minute or two untill the meat binds together itself...it is realy very quick.
If I add anything else it is generally just a sprinkle of fresh herbs ar a little very fine chopped chilli. I have on occasion added things like onion, mushroom,sweetpeppers but have always chopped them very fine and sauted them first to remove exess fluid.
The best burgers (so my family tell me ,as I am a vegetarian!!) are the plainest burgers with little more than a sprinkle of salt and pepper added to the meat.

I will try the cheese and bacon idea as I think that my husband will love that, is it nessersary to freeze it or will it chill firm in the refrigerator?

The cheese and bacon will make up for any issues with the lean beef.
Fridge is OK, frozen helps keep the shape better as there is lots of heat in your hands and it will soften things up. The trick is to completely seal the cheese within the burger so nothing leaks out and causes flare-ups. You may want to use a press and do 2 very thin patties and make a kind of ravioli with the filling.
I agree about simple, S&P s/b best, but I can't stop myself from adding garlic.

I always add a squirt of tomato ketchup and/or worcester sauce to mine and they taste devine.

Gourmet burgers 

Ingredients

1 small onion, finely chopped
2 garlic cloves, finely chopped
1 large pickled gherkin, finely chopped
500g/1lb 2oz minced beef
large bunch flat leaf parsley, chopped
1 large lemon, grated zest only
salt and freshly ground black pepper

Method
1. Combine all the ingredients in a large bowl and mix until well blended.
2. Shape the mixture into 4 burgers and chill until ready to barbecue.
3. Cook the burgers close to the coals for 3 minutes per side for rare or longer for medium to well done. Serve immediately.