Grade: Of course the grade is going to play a big role in the quality of the meat. Now technically a prime rib is prime grade and anything else is just a standing rib roast. These days, however, prime rib is a more general name and refers to the cut not necessarily the quality. Of course prime grade is very expensive and hard to get so look for a choice or select grade. Ask the butcher if you have a question. More important than all that, be a picky shopper. Look, it's your money. Choose a rib roast that has a bright color with milky white fat. Avoid dull colored meat and yellow fat. Also, look for even fat distribution and a good layer of fat around the ends. This isn't the time to look for the leaner cuts.
Trim: This is when you really want an experienced butcher. The perfect prime rib needs a very professional touch, because any professional will know not to touch it. The less trimming the better. You want all the bone and fat right where it is. Unless something is hanging off you don’t want to touch this roast any more than you have to. So no trimming.
Size does matter. It might seem backwards but larger roasts are actually easier to cook. Small roasts are less forgiving. Think about it this way. A small roast can go from perfect to ruined in a few minutes, but a larger roast will give you a bigger window of opportunity. As long as you can fit your roast where you are going to cook it (i.e. roasting pan, grill, smoker) you can go as big as you want. Smaller might seem easier, but it really isn't. I don’t recommend a rib roast under three bones.
Age: Aging beef is something of a lost art. Most markets want their inventory to move quickly and there are some risks involved with aging meat. If you have a butcher who ages meat then it is definitely worth the expense. If you are very brave and willing to risk everything then you can go a head and do it yourself. Aging meat concentrates flavor and improves the tenderness. It also takes as much as three weeks. Because the meat is exposed to bacterial growth there is always the chance that you might end up with a toxic mutant in the refrigerator, but if you've tried aged beef you'll never be satisfied with off the shelf meat again.
Rest: This is the biggest secret to the perfect prime rib. In fact it is the secret for most all meats. When the roast is nearly at the perfect temperature (as your trusty meat thermometer told you), it's time for it to rest. Remove the roast from the heat, cover, and let sit for about 15 minutes. This allows the meat to relax and tenderize. As the meat relaxes the juices flow back through the meat improving its flavor. I can't stress this enough. Take the roast out of the oven, put it on your carving board, cover, and set the timer. Fifteen minutes without a cut, a peek, or a prod. This will give you a great roast, so be patient.