Its very important that you keep your elbows in during the set. Your elbows will have a tendency to flare out as you move the weight, and you must keep them pointing forward.
Keep the rep timing slow and control the weight.
And finally, dont lock your elbows out at the top of the movement. This will take the tension off the triceps.
Instructions
The lying tricep extension (AKA skullcrusher) is one of the best tricep building exercises there is. Sit on the end of a flat bench with a barbell on your thighs.
Grip the barbell with an overhand grip (Palms facing down) with your hands about shoulder width apart.
Bring the bar up to your chest and lay down on your back. Extend your arms straight up above your chest.
Keeping your elbows fixed in place and not pointing out, slowly lower the bar until it is about an inch from your forehead (hence the name skullcrusher!).
Pause, and then slowly extend your arms back to the starting position.
Do not lock your elbows out, and then repeat for desired reps.