I have a lot to add to this discussion being from Texas, not to mention being an award winning brisket cook, but I'm going to take it one step at a time, to hopefully continue this thread and help as many struggling brisket cooks as possible. 9 times out of 10, if a cook has failed with brisket, it's because they didn't cook it long enough, but there are other factors at play too.
Several great points have already been made so far about meat quality, smoking temps, smokers, etc., hopefully I can just add to those along the way.
Let's start at the supermarket....
First of all, yes, Select grade briskets can be cooked to be both tender and juicy, but the better the quality of meat you start with, the better your chances are at a great result. First, you should know that briskets aren't individually graded, the actual carcass is graded at the rib eye section thus all the cuts that come off said carcass are graded the same, here is a good read for you on beef grading process thanks to our boys here at Texas A&M (my daughter is headed there in the Fall, and so are my bank accounts!)...
http://meat.tamu.edu/beefgrading/
So it is very possible to buy a Select grade brisket that has the characteristics of a Choice, or even a Prime. I look for flexible toe end (or flat section), I also look for internal marbling, not the fat stuck to the outside or the fat cap, but striations of fat running through the grains of meat in the flat section. The higher the content of internal marbling, the better the chance at a super moist end product. This is what makes the point end of a brisket almost a delicacy due to the high fat content. If you have yet to purchase a Prime grade brisket, please do before you knock the difference between Select, Choice and Prime. The texture, moisture and mouth feel are just night and day. Now that does not mean that you won't end up with a Prime that is no better than a Select, simply due to the grading process, but in my experience, the Primes have been well worth the money. Fat cap has little to do with the end product and how juicy it turns out, the following pic is how I trim up a brisket before a competition...
Very little fat left other than the bottom side. I want as much meat surface to be hit with my rub and smoke as possible. Basically I do what the supermarket calls a super trim, they just get to charge you more per pound for their effort. Another tip is to wet age your briskets for approximately 28 days, preferably from the kill date, but around here I usually can't determine the kill date, so I just do it from the date of purchase. Stick it in it's original packaging in an extra fridge or the back of your daily one and leave it untouched for 28 days, then either cook it that following weekend or store it in the freezer until you are ready to cook it.