Try brining for an hour before cooking. Mix a solution of equals parts cider vinegar and water. Then add kosher salt, brown sugar, garlic, pepper, and any other heb you like, such as rosemary. I learned this from Alton Brown, so search "Good Eats" for the exact proportions and ingredients.
A pork tenderloin will suck up a brine quickly, so be careful. Do it right, and it's some good chit!