When cooking, how do I thicken up a sauce into gravy?

As usual, I have a concoction on the stove. Meat, onions, mushrooms, peppers, Italian seasoning, can of golden mushroom soup and one can of beef broth - a little worchestershire and dijon for funkiness. It takes great! However, I want to put it over some ery mashed potatoes and my sauce is very runny. Every time I try to use flour, I get clumps and then your eating chunks of deep fried flour! YUCK! Please help me be a better cook.