Best way is to make a roux which is mixing the flour very well with butter so that you have a paste. This way it doesn't clump, or you can use corn starch mixed with a little water. Both ways will give you creamy smooth gravy. I use a good wisk while adding either.

I agree with Tickle. I make a paste and keep adding broth until it is thin. Then I gradually add it to the hot broth stirring the whole time. I also use cornstarch or Wondra rather than flour.
When I add it to cold broth it tends to lump.

I take my flour and mix it with milk or water(depending on the type of gravy) and whisk it until it is smooth then I bring the meat broth to a boil and whisk in the flour mixture, it thickens while it boils. This also works using corn starch.

What I usually do is put some of the juices into a large coffee cup and add flour a bit at a time and whisk with a fork until all lumps are gone. It's easier to get the lumps out of a small ammt, and then you can pour the whole thing back into the juices, and let it do it's thing.

Or you can do it the opposite way. Make a rue with equal parts butter and flour and let it cook down for a few minutes to get the floury taste out (stirring constantly) and then add your juices to the rue. That way there are never any lumps at all.