Paul,
Congratulations on your first brisket! I'm not an expert at BBQ by any stretch of the imagination, but I've experienced drier-than-expected brisket and actively sought solution. I finally settled upon a methodology that seems to have worked, so I've stuck with it. This methodology isn't unique to me, but a culmination of several methodolgies that I've seen or heard about that seem to make sense.
I have a remote thermometer (Maverick), so I have a fairly good idea when the internal temperature of the brisket has broken plateau (variable... but usually around 160 to 165). As soon as I see the temperatures begin to climb, indicative of breaking plateau, I remove the brisket from the WSM for a bit of preparation prior to foiling. At this point, I set the brisket on foil and apply the final sauce (if used). I also douse the brisket in the mop (if used). It's generally a mop or sauce or both... never nothing.
I then wrap the brisket tightly, puncture the foil with the temperature probe, and return it to the WSM for it's final trip to 190F. I pull the brisket at 190 and try to wrap it in towels and place it in a cooler for a minimum of 3 to 4 hours. I recently bought a Coleman "Ultimate Extreme" cooler and found that I can keep brisket within the 'safe zone' for much longer than 4 hours. I try to honor the cooler stay time, doused in moisture, as best I can. It seems to really make a difference.
Before slicing, I take the brisket out of the cooler and place on a cooling rack on the counter a full 1/2 hour minimum before unwrapping and slicing.
One other personal observation: Out of necessity, I once shut the WSM down (closed the vents) as the temperature of the brisket was climbing to the desired 190. When we returned, I found that the brisket temperature was 193 and the smoker temperature was 185, so I wasn't too concerned. I theorize that the brisket had been above 190 for a couple of hours.
I followed my standard protocol with this one exception. After slicing, I found that the brisket wasn't dry... but it wasn't moist either. Just another lessoned learned.
Our family and guests still managed to eat quite well that night, so I'm sure it wasn't too bad.
As a final note, the briskets I buy locally are "USDA Choice". They have a resonable amount of marbled fat which may contribute to the final moisture. Since I haven't used any other brisket grade, I can't provide any comparison, nor can I validate my recommenation.
Have fun with your smoker! I know that I do
