Once you have the tenderloin trimmed and ready prepare a brine. You will need about a quart of water with 2-3 tablespoons of plain or kosher salt. Place the tenderloin in the brine and refrigerate for about 12 hours.
To enhance the flavor of the tenderloin, use a rub, wet or dry. This can be a simple coating of garlic and pepper or whatever you prefer for pork. While the brine helps to add moisture to the meat the rub will add flavor and create a crust over the surface while it grills.
The third trick in getting your tenderloin just right is in the grilling. Once the tenderloin has been removed from the brine, dried off and coated with a rub it's time to put it on the grill. Have your grill good and hot; a double layer of coals for charcoal or the high setting on your gas grill. Now the real trick here is to treat the tenderloin like it has four sides. Being a long round roast you need to get the insides cooked through without burning the surface. Grill the tenderloin for about 3 to 4 minutes on each of its four sides.
The last thing to do is remove the tenderloin from the grill when its internal temperature reaches 145 degrees F. Of course this is a little low for pork, so what you do is cover the tenderloin with foil and let it stand for about 5 minutes. Because grilling uses a very high heat food will continue to cook after they are removed from the grill. Use this to let the tenderloin reach around 150o before you serve it. Make sure that you place the resting tenderloin in a spot where it won't be artificially cooled.
Once done, slice the tenderloin about 1/2 inch thick and serve.