Skull crushes are great for building mass in the back of your arms and isolate the triceps.
Lie back on a flat bench with an EZ curl bar resting on your thighs. Kick the bar up to your chest and then extend your arms over your chest; like a bench press. Next, bend at your elbows and slowly lower the bar until it touches your forehead. Make sure to keep your elbows in and do not allow the bar fall onto your face. Your upper arm should not move throughout this movement and your elbow should be the pivot point. Next, squeeze your triceps and push the weight back up to the starting position. Repeat this movement until you complete your set.
Make sure to use a weight that you can handle easily; you do not want the weight to slam into your forehead. Also, make sure to feel a stretch at the bottom position.
This same exercise can be performed using a straight bar, or done on a decline bench press; it should be done exactly the same as described above.