I don't usually inject brisket, but when I do I just use Beef Stock with a little Worcestershire. I prefer the taste of the beef, over an real flavoring from an injection. I have a Great Outdoors Smoky Mountain smoker, and the brisket goes on the shelf fat cap up. After the first 2 hours I squirt it down every hour with beef broth, with a little bourbon.
Be prepared to wait. A lot of the briskets I have done hit 2 plateaus, so the temps will stick for a while. I did a 19lb whole brisket that took 22 hours @250 degrees that actually hit 3 plateaus, but man when I took it off (after a couple hours rest in a cooler) it sliced like butter. Brisket, more so than any other meat takes Patience, but the results are well worth it. Also make sure you cut it against the grain. If you are doing a whole brisket, the grain may change on you a few times, so keep an eye on it while you are cutting it.