The best results come from a "smoker" type grill that allows you to let the meat cook at a low temperature for a long time, just as you would do if cooking in the oven. I usually cook mine for about an hour per pound. It depends on whether I get an untrimmed or fully trimmed brisket. In my opinion, untrimmed are best for longer periods of cooking. Barring that, just make sure you use the indirect method of cooking, i.e., don't put the meat over the hottest part because you will dry it out for sure. Put a pan of water underneath to catch the drippings. This prevents the fire from flaring up as the grease drips down. One method I've used in the past is to set the brisket on top of aluminum foil with the edges turned up. This protects the meat from the heat as well as provides moisture for self-basting while cooking.

Good Luck