some of the most common ingredients are paprika, onion powder and garlic powder. the rations of these will depend on your tastes.
most also add some salt, again, to taste. some leave it out.
some add sugar, either white, brown or raw, to balance the savory flavors. if you do this, i recommend raw sugar as this is supposed to help keep the meat and the bark from geting a burned look and taste.
pepper is a good addition - some add cayenne pepper to taste. this is a good thing but be careful with it.
many toher herbs and spices, including cumin, oregano, thyme, lemon, and honestly, too many to mention. those should be some basics, however.