Here's a few tips for making skullcrushers most effective:
1.) Lay flat on a bench with your head just off the end. You can have your feet flat on the floor or bent and feet on the bench; it's a matter of preference.
2.) Find the most comfortable hand placement on a curl bar. 8-10 inches is probably best for most people, or in other words, the inside grip (that's what I always called it).
3.) When you press the weight up, keep your arms locked out, but not directly over your head. Instead, keep your arms at about a 45 degree angle (back behind the top of the head) at the peak of the movement. This keeps the triceps contracting, having to fight the resistance of gravity versus the elbows taking most of the stress if you're locking out completely perpendicular to the ground. If the 45 degree angle just feels too uncomfortable for some reason, play around with it. But remember that if you lock out completely vertical, your triceps will be able to rest as your elbows take over. The key is to get a full extension while keeping the triceps fighting the resistance of gravity the whole time. This makes the lift harder, but that's what you want!
4.) Although some movement of the elbows is inevitable, keep them as stationary as possible as you lower the weight back behind the top of the head, getting a full stretch. Bring the weight only down to your forehead limits range of motion. Press the weight back up to the starting position, keeping the stress on the triceps the whole time by not locking out vertical. Think about what the triceps are doing throughout every rep during the entire set.
5.) Keep your elbows tucked in, rather than flaring them out.
6.) Try doing skullcrushers on a decline bench to mix things up.
7.) There is a potential superset built right into skullcrushers: close-grip bench presses.