this is certainly heresy, and it may be unrelated, but my tri's have gotten real defined lately and i think it's because of upright rows with a barbell. i know that's an upper back exercise but i think that it's hitting some of the secondary muscles of the tricep or something. it certainly has pushed that nub or whatever it's called out on the side of my arm
i'm doing 3 sets of 10 with 90 pounds.
it's a good back exercise, regardless. give it a whirl, see if it makes a difference.
another tri exercise to kill the boredom is the rearward extension. lean over a flat bench with same knee as working arm on bench and grip forward with opposite hand. have the weight arm perpendicular to your body, down by your knee on the bench, and bring the weight up behind you until your forearm's parallel with the floor.
basically: H for head, W for weight, B for bench
H-----|--
| | | | W | \----|
BBBBBBBBBBBBBBBBB
to
H------------W
| | | | \----|
BBBBBBBBBBBBBBBBB
hurray for ascii art.
[edit: screw it, stupid formatting - you get the point]