How do you get braised beef brisket to remain juicy?!

Hope you read this before you actually got your brisket.

I've been smoking, grilling, braising meats, etc. for over 20 years. If you are braising, then fat and liquid are key.

Brisket is a tough cut and benefits from a long, slow cook.
I always ask my butcher for an untrimmed brisket, as the ones in the meat display case are trimmed of fat.
The extra fat automatically bastes the meat as you smoke, grill, braise, whatever, and will deliver to you a much juicier, more tender and more flavorful pierce of meat.

Professional Chef and restaurant owner.

Brisket is one of those meats that you have to cook for a VERY long time, at very low heat. The best recipes I have used call for cooking the meat at 200-225 degrees for 8 hours. Of course, for it to be a brqaise, you must cook it in braising liquid, just enohg to covme up about 1/2 way up the side of the meat, and seal your cooking vessel as best as you can.
If you cook brisket at high heat, it will become tough and dry, no matter how much liquid is retained in the cooking vessel. You need to be able to break down the tough connective tissue between the muscle fibers in a cut like brisket, and you can't do that properly with high heat. Low and slow is the way to go.

Always cook it low & slow. Remember to keep mopping it with a basic mop mix (google it), then keep it covered with foil, as already suggested.

There are many recipes and methods for low & slow cooking of brisket. Google it for more tips.

You have to serve it with some of the braising liquid or else it will go dry immediately

Cover whatever you cook it in with tin foil to keep moisture in.