Never tried injecting after the meat had started cooking like TXSmoker's method, but I do agree with Humdinger that the rest is a key factor in keeping a brisket juicy and moist. However, the biggest factor in a briskets juiciness is the internal fat content or marbling as it is referred to. Not the external fat (external fat does nothing for flavor or assists in anyway in keeping the meat juicy other than acting as a shield from direct heat), but it's the internal fat that lends a hand to both a juicy and flavorful piece of meat. Also, remember that buying a Choice or Prime grade brisket does not guarantee the internal fat due to the fact that the cow the brisket came off of was graded Choice or Prime, not necessarily the brisket itself. Unless I am wrong, it is the rib eye cut thats inspected for the grading process, therefore it is possible to get a brisket that would be considered Choice grade off from a cow whose ribeye was graded select and vice versa. Another tip for keeping the flat juicy is when you slice it. All protein, once exposed to air, will dry out fairly quickly especially when piping hot, the optimal slicing temp for a cooked brisket is under 170 IT, so that is at the very least an hour long rest if not 3 to 4 depending on how you rest the meat (I rest mine like most, in a cooler, wrapped in towels). Consider that most briskets are toothpick tender anywhere above 200 degrees IT, so you have to allow the meat to return to 170 or below slowly, not rapidly, to allow the muscle fibers to relax naturally, allowing redistributon of the juices throughout the meat. The following is my suggestion for great brisket everytime, as TXSmoker proved, there are more than 1 right way to cook a brisket, the trick is finding what you and the people you cook for like. Most of all, read the last sentence carefully, have fun and be patient. Briskets take lots of patience!
In general, 225 to 250 pit temp works just fine, smoke with your choice of any number of woods, oak, hickory, pecan, mesquite, cherry, apple or a mixture works well. Fat side up or fat side down, everybody has their own way, but the most important factor is your smoker, try and place the brisket on the smoker where the meats surface is protected from the heat source, using the fat cap as a shield from direct heat on the meat surface. Smoke for 4 to 6 hours until the IT reaches 160 to 165 or you get that nice, mahogany color to the meats surface. Wrap in foil or butcher paper your choice once the IT has reached 160 (if your not a wrap guy, at the very least pan it at this point to catch and save the precious juices that will begin to come from the meat as it enters the stall, these juices will come in handy later for vac sealing leftovers or if your brisket turns out dry), continue the cook until the meat reaches 190 IT, at this point start probing the flat section (thin, lean end) every 30 minutes or 5 degrees of IT increase, until a toothpick or probe can be slid in easily, like probing a tub of butter. In my experience, most briskets won’t become probe tender until around 205 IT, but everyone cooks a bit different. Once you have reached this point, pull the brisket from the pit, vent the foil or other wrap for about 10 to 15 minutes to allow the cooking to stop and the excess steam to escape, wrap in foil or foil pan if it is not already, place in a cooler wrapped in towels for at least an hour, 2 or more is better. Once the IT of the meat has dropped down to 160 or below (optimal temp for the juices to have been redistributed through the meat), it is time to slice it and enjoy. If your slices tend to be a bit dry, simply dip in the au jus or place the slices in a pan and drizzle the au jus over the slices, this will both enhance the flavor and assist with the moisture.
Have fun and most all have patience!
