This article is from the Fitness FAQ, by Jeff Gleixner (glex@cray.com) with numerous contributions by others.
Basic Form: Work triceps with slow movements to reduce the stress on
the elbows. Try to keep your biceps slightly flexed.
CLOSE GRIP BENCH
Set-up: Assume a stable position on a flat bench. Feet flat on
the floor, butt, back, rear deltoids and head on bench. Grasp
barbell with a narrow (6 inch) grip.
Movement: Lower barbell to sternum, keeping elbows near rib cage.
Press bar upward using the triceps. Make it feel as if the
weight is pressed slightly toward the feet.
Variations: Can be done using dumbbells and/or on a decline bench.
Using a Reverse grip (palms facing biceps instead of triceps.
Underhand), with a shoulder width grip. will also hit the Triceps.
Muscles Worked: Triceps (Medial head), Shoulders (front delts),
Pectorals (inner chest)
CABLE PUSHDOWNS
Set-up: Take a comfortable position in front of a high pully.
Grab bar with a narrow, thumbless grip. Pull weight until
elbows are tight against rib cage and hands are around the
upper chest, knees slightly bent.
Movement: Keeping your elbows and upper body motionless press the
bar in a slight arc until the elbows are comfortable locked and
triceps are contracted. Slowly resist the weight as it is
returned to the starting position. The elbows are the pivot point,
Don't cheat by swinging the weight or upperbody.
Variations: The best "bar" is a rope handle, avoid using a long
straight bar, use a V-shaped bar if rope isn't available. Can
also be done with elbows spread out to the sides and the upper
body bent forward so the weight is being pushed down in a straight
line. If using rope handle flare hands out at the bottom position.
Muscles Worked: Tricep (Outer head)
OVERHEAD DUMBBELL EXTENSIONS
Set-up: Stand with feet about a shoulder width apart, and a
dumbbell resting on your shoulder with your elbow pointing
straight up and your biceps flexed. The dumbbell is held
with a regular "curl" grip. The starting position looks like
the weight is being used to scratch the upper back.
Movement: Without moving the upper arm, lift the weight, rotating
the hand so the palm faces forward, until the dumbbell is
above the elbow.
Variations: Two handed dumbbell extension, where the weight of the
dumbbell is being supported by the palms of both hands, thumbs
wrapped around the bar. Using a barbell or EZ-curl bar or a low
cable pully with a rope handle. One-arm extensions give a wider
range of motion.
Muscles Worked: Triceps (inner & medial heads)
 
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