-Flat or decline lying 2-hand extensions behind the head, lowered below the bench. OK to cheat just slightly at the bottom point, towards the end of the set to get the weight moving. Don't worry about elbow positioning. Most effective are hammer bar, super e-z curl bar and 2 hands around the end of one dumbbell.
Can lead to elbow problems. Keep the reps 8-12, rests between sets no longer than a minute, and do another compound tri exercise before extensions: dips, bench dips, close-grips or pushdowns. Changing from one bar to another, for example from a straight bar to a hammer bar, can also make a big difference in reducing elbow pain.
Pulleys won't hurt the elbows and create a similar if not better effect:
-Kneeling extensions using a high pulley and rope/v-bar:
http://www.bodybuilding.com/fun/exercises.php?Name=Kneeling+Cable+Triceps+Extension-Bent-over extensions using a high pulley and rope/v-bar: same as kneeling extensions but without the bench. Bend over to the point that the upper body is parallel to the floor.
-Lying extensions using a low pulley & rope or v-bar attachment. Try both a flat bench and a slight incline.