My Mom makes the most fantastic turkey burgers. Her secret is Lipton Onion Soup Mix.
The only other added ingredient was cayenne pepper.
She wouldn't add salt because the soup mix has plenty.
Once the burger is cooked (she pan fries them), she deglazes the pan with a bit of water and then "bastes" the burgers with the pan drippings. Then she melts a slice of provolone cheese on top and serves the burger on a buttered, toasted Kaiser roll with thinly-sliced onion, lettuce, and honey-dijon mustard.
If you don't like the ingredients in the soup mix (it has MSG and a couple of disodiums), try this: http://www.food.com/recipe/copycat-liptons-onion-soup-mix-24952
I rather suspect it's the MSG and disodiums (all of which are recognized as safe for consumption, but demonized by foodies everywhere) that make them taste so great.