Students of anatomy and kinesiology might have an advantage over the rest of us when it comes to triceps training. Not only do they they know the names of all three heads (lateral, long, and medial), but they also know how each head moves. That means they can figure out the best ways to target each one. But you don't have to take an anatomy class to know what they know.
People tend to think that the three triceps muscles originate at or near the shoulder and attach at the elbow joint. That's close, but not close enough—especially when it comes to the long head. This bulky innermost head originates on the shoulder blade (scapula). You can fully stretch it only when you position your arms overhead during an exercise like overhead extensions.
That's important, because a fully stretched muscle is capable of a stronger contraction. If you always keep your arms by your sides when you do triceps exercises, you aren't going to activate your long head nearly as much.
To work the long head, think overhead movements, such as skullcrushers. Lying on a bench with your upper arms perpendicular to your body and the floor, you're getting the full impact of gravity. You can even shift a bit more emphasis to the long head by doing the move on an incline bench, which positions your arms in a more overhead position. (Same thing goes when you do close-grip bench presses on an incline bench.)
So when you're putting together your next triceps routine, make sure you add overhead exercises that really work the long head. Keep doing your 1-2 multijoint exercises and arms-by-your-sides movements like press-downs and kick-backs. But throw overhead moves like skullcrushers into the mix, and you're on track for maximum arm girth. Boom!