If the leg of lamb is boned, you can keep it from getting all dried out by simply rolling and tying it. Cut off most of the external fat. Slice it open the rest of the way so the meat will lie (sort of) flat, and rub it with olive oil. Sprinkle with salt, pepper, and rosemary inside and out.
Roll and tie the meat, attempting to keep the roll as even as possible. Don't worry about it too much if it isn't completely even; this will just help you to obtain slices in a range of doneness. Slice up some cloves of garlic and make cuts into the roll to insert them.
What I like to do is to start with a high temperature like 450. Put some water into the bottom of a roasting pan with a rack and place the lamb into the oven on the lowest position. Roast for about 10 minutes and then flip the rolled roast. Roast for another 10 minutes, and then lower the temperature to 325 to complete the roasting time.
If you roast your boned leg of lamb at 325, it usually takes around 25 to 30 minutes per pound to reach an internal temperature of 145. Lamb is medium rare when the internal temperature reaches 145; this also happens to be the safe temperature for killing bacteria in whole muscle cuts of lamb. An internal temperature of 160 gives you medium doneness, and is the safe temperature for ground lamb as well.
You can usually take the lamb out of the oven when the temperature reaches 140 and it will climb the rest of the way as the meat rests. Definitely do let it rest, as this results in the juices being redistributed. Cover it with foil to keep the outside from cooling off too much.