The use of barbecue sauce during the cooking process is probably the most misunderstood techniques used in cooking outdoors by the backyard cook. Tomatoes and sugar, some of the primary components found in barbecue sauce, both have low burning temperatures. That means that they burn very easily and quickly.
The misconception is that placing a lot of barbecue sauce on the meat helps keeps the meat moist and tender. The truth of the matter is that you're building up a charred crust of sugar and tomatoes.
A better process would be the use of a basting sauce (a recipe for which can be found in the recipe section of the site) during the cooking process. Just before you remove the meat from the grill, and while it is not directly over the heat of the grill, place the barbecue sauce on all sides of the meat and allow it to sit (away from the direct heat) in the pit for 15 to 20 minutes. This sets up a nice tender surface and prevents the sauce from being burned. The difference in these techniques will make you the hit of the neighborhood!