Cooking a brisket to 100% perfection on a smoker can be tricky. If you're not careful, part or all of it can dry out. You can take all the necessary steps to prepare the meat and mop frequently but a lot depends on HOW YOU PLACE the slab of meat on your smoker. The most important aspect is to know how your smoker convects it's radiant heat. Radiant heat travels in a straight line. Anything it hits will be heated and that starts the drying process.
You have a couple of choices -- FAT CAP DOWN or FAT CAP UP and additionally, into the cook, you can FLIP or TURN.
Which way produces the best results? The answer is if the one you're using gives you the results you want then why change. I wasn't satisfied with my brisket and had to experiment.
FAT SIDE UP: This was my first method of choice. I felt the fat from the cap would keep the meat moist. It didn't -- all that happened was the fat washed away my rub. I used to use mustard as a binder and that made matters worse because mustard is water based and will not dissolve the rub, which are oil soluable, as well as veggie oil which is a better choice because it has a higher "smoke point" than olive oil.
FAT CAP DOWN: The fat cap acts as a shield. It shields the meat from the intense heat. Personally, for me, it produces a more tender result with less drying because the flow of air (convection) around the meat from top to bottom is more consistent.
FLIPPING: I obtained some good results with this method because one side had a chance to rest but the end result was more drying than I wanted. When you flip the meat you're forcing pooled moisture to fall out. Instead of flipping, I obtained better moist meat results by heavy mopping.
TURNING: If you allow the meat to remain in one position, one side is going to dry out due to uneven heat flow even if you mop it to death. Turning is like flipping except that the pooled moisture doesn't fall out.
So, what's the best way for ME cook a brisket? Here's what works best for me. I cook on a Lang. On a Lang the heat radiates from the bottom and from one side -- when the heat makes the turn around the reverse flow plate it makes a bee line for the chimney. To deal with this, I place the meat on a grate over a water pan and cook with the fat cap down. Every hour or so I baste & turn the meat. I maintain a cook temperature about 260*. I let the meat rest in a beer cooler for about 30 minutes prior to carving.