Trial and error, rarely error but it is all about finding what works for your taste buds. Personally I like to tweak the injections and play with alternative rubs to add to base injection methods. That's just me, trying to find the holy grail of profiles, it's part of the bbq love. A funny instance last year, I was in a comp and we were also vending. We had people coming back to us telling us that our pulled pork was the best they had ever had, just flat out mind blown about the flavors, asking what we put on it.
I chuckled to myself and thought "we are over thinking bbq". All we did for the public consumption butts was toss them on the WSM full of Kingsford. No rub, no injection, nothing other than the natural swine sweetness it produced itself. Sauced with Sweet Baby Rays. Can't get any more basic than that.
Brown Chicken Brown Cow, North Idaho http://www.facebook.com/BC2BBQ