Here's a simple pork rub from Cook's Illustrated's Best of Barbeque book. I have used it with great success. I use mustard before I apply the rub & I usually spritz with apple juice half way through & the bark is fantastic!
(Makes about 1 cup)
4 TBS Sweet Paprika
2 TBS Chili Powder
2 TBS Cumin
2 TBS Brown Sugar
2 TBS Salt
1 TBS Oregano
1 TBS Sugar
1 TBS Black Pepper
1 TBS White Pepper
1 tsp Cayenne Pepper