Hold a barbell or EZ curl bar with a relatively narrow grip overhead. Keeping the elbows as high as possible, bend the arms to lower the bar as far as you can, then lift into full extension, again trying to keep the elbows high instead of driving them forward.
 
Purpose
The overhead barbell tricep extension strengths all three heads of the tricep, but emphasizes the long and medial head over the lateral head.
 
Programming
Generally sets of 6-15 are appropriate.