First off, brisket flat is unquestionable the easiest thing to screw up there is. It totally lean and relatively thin. It needs tons of time to break down the connective tissues. That is the key, that turning of the collagen in the meat to a jelly like consistency which gives the meat that unctuousness that we all crave. That is achieved in a very small window of cooking time. Generally folks talk about that being the temperature range of 195-205* IT. But dude, listen to me, that means diddly squat. What you are looking for is probe tenderness, like a toothpick going into jello. It wobbles when you shake it. Before and beyond its raw or dry shoe leather.
There's also the rest, as has been stated. It needs to rest at least an hour but preferably longer so that the meat can finish its cooking process slowly. If you just yank it from the cooker and begin carving it will cave on you.
These are probably the two most critical steps that a lot of first time guys completely miss. You might also read a little bit about injecting, especially in a flat. Some guys also cook in a foil pan (which I believe constitutes braising but other might disagree). I wrap in pink butchers paper at the stall to avoid messing up the bark it took me 12+ hours to achieve.
Bottom line, keep trying. Continue to read posts that talk about steps to successful brisket. You will eventually get it.
