Flats can be tough to nail. Ask a butcher if he can get you "packer trimmed" briskets. If the butcher is trimming them himself, tell him to leave at least 1/4" of fat on top.
The reason packers are better is that the point is still atached. Thus, there is a large ribbon of fat running inbetween the flat and the point. This is what gets the flat to be so tasty.