Susan - great exercise! You're absolutely right to drop your weight and be sure you've got the form right. As with any triceps exercise, good form is the KEY to getting results.
Hmmm, these are the things I concentrate on when doing lying extensions (I'm guessing you're using DBs?):
- the starting position is both arms straight up in the air (if using a BB, hands are about 4 - 6 inches apart)
- your elbow is a hinge - the only part of your arm that moves is from the elbow to your hand - the upper part of your arm always remains stationary
- keep elbows IN by your head and pointed at the ceiling at all times
- you only need to bend your arms to a 90 degree angle with DBs - parallel to the bench
- at the top of the movement, straighten your arms (it's OK to lock out your elbows) and SQUEEZE your triceps
a little video of DB extensions that may help. If you're doing BB extensions (a/k/a skullcrushers), here's
I'm sure you'll get some more tips from the others.