The secret to a nice moist pork loin is not to over cook it. Todays commercial pigs are much cleaner and healthier so you don't have to cook it to well done like our grandmothers did. I usually cook to about 140 internal temp then let it rest for about 10 minutes tented with foil letting carryover cooking finish it off.
This recipe is from the Naked Chef it calls for pork chops but I always do it with a pork loin. Works with boneless as well as bone in. I just put everything in the oven at the same time, unless the pork roast is bigger than 3 lb, then I roast the meat for about 30 to 40 minutes and add the veggies.
http://www.foodandwine.com/recipes/p...s-and-potatoes