Make sure that the lamb is fairly straight in the hip. If it is too straight, then move on to another lamb. Choose a lamb that has an angular shape coming out of the shoulders towards the hindsaddle. The lamb should have an overall triangular shape. A raised edge on the outer edges of the loin is desirable.
Leg conformation is also important.
When you look at the lamb from a front view you want to see nice, straight legs, not ones that bow out or where the knees knock together. The toes should also point forwards, not toed out or in. From the rear view, again you are looking for straightness. The side view also tells you something about the conformation. When you examine the front legs from the side, you don't want the knees to stick out forwards or be bent in the opposite direction towards the hind end.