The triceps to a lot of heavy work and the work load on the tricep from pressing motions involving the shoulders and chest is a lot. For tricep exercises, the weights should be moderate. Really concentrate on the form, do not go very heavy. Squeeze and lock out the tricep at the top of every repetition. And the best exercises for triceps in my opinion are these:
Close grip bench press
It is very similar to the bench press, only a narrower grip is used to take away the tension from the chest and put it on the triceps. It is a great mass builder for the triceps. Your hands should be about 12 inches apart. Lower the weight, keeping your elbows directly under and in line with the path of the bar.
Tricep push downs
Keep the hands 6-12 inches apart and use a small straight bar. Push down and then also lock down. Hold and squeeze the weight for just a split second at full extension. You can also to this exercise one arm at a time using a rope. This method allows you to contract the tricep fully as the muscle is in a natural position to be contracted, when the palm faces the body in a locked position.
Lower the weight to your hairline at your forehead, using an e-z bar. Return the bar to its starting position, lock out and do not forget to tighten the triceps.
Only perform modified dips to keep the pressure on the tricep. Start the exercise with a locked out position. Using the triceps lower your body about 4-5 inches. Then return to the original locked out position. Use as little momentum as possible.