My best advice is just to experiment. I got mine a couple of years ago and love it. I typically just make a rub out of random spices. Never the same twice, but a good starting point is:
Brown Sugar
Black Pepper
Garlic Powder/Garlic Salt
Paprika
Oregano
Red Pepper
I usually mix these (its mostly brown sugar and Paprika) ahead of time (just eyeball it), rub it on the roast, and let it sit overnight in the fridge. You'll learn to know what you do or don't like in it.
The key I think is the brown sugar. Gives it a sweet taste mixed with the smoke. MMMMM this is making me hungry.