Use a BBQ rub of spices without too much salt. That's because salt attracts moisture and moisture attracts salt. This means that if you put a lot of salt on your meat just before cooking, the moisture in the meat is going want to move to the salt and out of the meat.
Salt may dry out your food while cooking but you can do it right. If you want your meat to have a salty flavor and be moist, soak it in a brine solution for a few hours before smoking it. This will cause the salt to penetrate inside the meat and therefore hold the moisture in while it is cooking on the grill or in the smoker.
Rub spices on meat with your hands before cooking and let stand for about 1/2 hour. This will let the flavor of the spices penetrate the meat and stabilize a bit before it enters the barbecue grill. It will also let the meat dry out on the outside so that the spices don't run off during the smoking process. You can add spices to your basting solution and baste the food to keep the moisture in while adding flavor.