You NEED some sort of lying triceps extension in there (i.e.: skull crusher). The long head of the triceps make up about 1/3 of the overall arm girth. The long head is also not fully recruited during typical pressing exercises as the short and medial heads are. We can talk more about the ins and outs of this, but basically in a successful skull crusher your upper arm will be fixed in place with the shoulder flexed to 90 degrees (i.e.: straight up and down) and the elbows must stay in line with the shoulders the whole time. You can do these with a variety of resistance, barbells, cambered bar, dumbbells, or cables. But, if your goal is big arms, they are a must.
Jason Cholewa, Ph.D., CSCS