Two Zone Cooking
Those that have cooked or smoked large cuts of meats or meats that require a long cook time are familiar with two zone cooking. Some of the meats that fall into are pork butts, briskets, ribs etc. The reason for two zone cooking is to keep the meat out of direct contact with the fire eliminating over cooking the exterior and under cooking the interior. With this technique you are basically like creating an oven.
If you have a gas grill-
If you have gas grill with 3 burners light one and keep two off and 4 or more turn off half. On the unlit side place a foil pan with water under the grate. Light the other burner(s) and bring the grill up to your desire temperature. Place the meat on the unlit side over the water pan. The water pan does a couple things. Water is a great absorber of heat therefore it will help to maintain the grill at a lower temperature which can be difficult sometimes with a gas grill. Also the water will help to create some humidity in the grill which can aid in keeping the meat moist.
If you have a charcoal grill-
Use one half of the grill for the fire and the other half for a water pan. Fire up the coals, bring to up to temp and place the meat on the unlit side. Basically the same principle as above with a gas grill.