Many articles appear on triceps training presenting exercises that are touted as being the best for developing the muscle. But if you take a look at these relatively common exercises, such as the triceps push-down cable overhead elbow extension and lying triceps extensions, you will see that they emphasize the pronated grip, that targets mainly the medial head.
Only the reverse grip pull-down targets the lateral head. The triceps kickback and rope pulldown, which use a neutral grip, target mainly the long head. However, in the kickback, to fully involve the long head it is necessary to execute it in two parts. The initial position is the same with the upper arm alongside your body, and the forearm hanging straight down. You then straighten the arm and when it is straight, hold it locked and then raise as high as possible above the level of the back. Do not rotate the shoulders to do this. You will feel an intense contraction of the long head when you raise the arm above the level position. Stopping at the level position does not target the muscle as strongly. This experience can also be done beginning with a straight arm and raising it above the level of the back. Another good exercise for the long head is the single arm overhead press using a neutral grip.
To truly target all three heads of the triceps you must use the three different grips — pronated, supinated and neutral. Do not get locked into only a few exercises that use mainly the pronated grip or a position in between the pronated and the neutral, as when using an E-Z curl or angled bar in a triceps push-down.
See Kinesiology of Exercise for more information on triceps exercises.