Standing Dumbbell Triceps Extension

Trains the Long Head.

How to do it

Stand holding a single dumbbell in both hands, or one in each hand.
Press it overhead until the arms are straight, then set the upper arms beside your ears.
Brace your stomach and keep your ribs down.
Bend the elbows to lower the weight behind your head.
Go down until you feel the stretch along the back of the upper arm, keeping the elbows pointing forward.
Straighten the arms back overhead without letting the elbows flare out.

Letting the elbows flare out to the sides. Elbows pointing forward keep the load along the arm; flaring them turns the movement into a partial press and shortens the range.
Arching the lower back as the weight goes behind the head. Ribs down and stomach braced is what keeps the spine out of it; the arch appears on the repetition where the shoulders run out of range.
Letting the upper arms drift forward. They stay beside the ears — once they travel forward, the long head is no longer stretched at the shoulder and the position that makes overhead work worthwhile is gone.
Going heavy. The weight is behind your head with the elbows bent, which is the least controllable position in any triceps exercise, and it is a poor place to discover the load was ambitious.