Many bodybuilders are trapped under the assumption that you have to either train for mass, or you have to train for definition. They will split their training year into these two sections. During the "mass at all costs" phase, they will pig out and sleep all the time. They will spend nine months doing nothing but mass movement for their body parts. They'll keep the weight very heavy and the repetitions very low. This will lead to large, bulky muscles without a lot of shape.
Then, the pre-contest phase will come around and they will completely abandon the tools that helped them to build that great level of mass. They will suddenly drop all the mass-building movements and run to the cables and machines for their workouts. They'll no longer train heavy for 6 to 10 repetitions. They'll cut the weight and "pump" their way up to twenty or more repetitions. They'll run fat burners such as ECA, and average two less hours of sleep per night. The result will be a physique that very quickly loses a great deal of muscle mass. Sure, you'll show some new cuts and definition, but complete abandonment of all those effective heavy compound movements will lead to smaller muscles. Then it won't matter how ripped you are - you won't be able to hold your own against the big boys!
In terms of the training any body part, but particularly the triceps, you must utilize a nice combination of heavy mass building movements, along with isolation exercises. Your repetition range should also vary from low (with heavy weights) to high (with moderate weights). And you'll want to include repetition selection which allows you to use low and high reps for both the compound and isolation exercises.
The exercises you use should include close-grip bench press, skull crushers, and dumbbell presses behind the neck as your compound movements. At least two of these exercises should be included in every workout. Your options for the isolation exercises will be much greater. Dumbbell kickbacks, triceps cable pressdowns, as well as a variety of triceps machines are all useful for delivering a pump to the muscle group.
Remember that it is acceptable to train very heavy at times, and very light at other times. In order to deliver growth, the triceps must be faced with heavy exercises. They are imperative for mass and the underlying shape which makes the triceps look three- or even four-dimensional in all lights. At the same time, your year-round training needs to incorporate some isolation movements. You'll need the patchwork of veins running through the horseshoe muscles of the upper arm, and you don't get that with low repetitions. You'll need to move a great deal of blood to the triceps, and heavy weights - which great for strength - don't do this. Mix it up!
The key is balance. You need to find it in all aspect of your bodybuilding, including diet, training, and supplementation. Going too far either way, to heavy or isolation movements, will leave you with an upper arm that is without balance. And this isn't going to win you any bodybuilding shows. Keep the training well mixed, and your triceps will grow symmetrically.