Remember, triceps make up about 2/3 of your arms total mass. Big biceps are nice eye candy, but big tris are what make the shirts tight. Anyway, my arms are my best body part. Genetics are important for arms unfortunatly, but that doesn't mean you can't get big arms, you just might not get the nice bicep peak etc.. I focus(well did before starting my 5x5) a whole day on bicep/triceps. Don't work triceps the same day as bench, one will suffer.
The key to big arms is being able to focus on the muscle and let that muscle do most of the work. I am also big on cheating to get the last rep. Bend your back a little to finish that last curl if needed, or open up your elbows a bit for tri exercises. That last rep is what you work the whole set for don't puss out. Use heavy weights a lot. Doing 15 reps doesn't seem to work for me to build mass. I like the 4-8 rep range, more or less on light or heavy days. Also, you can superset some sets as you finish the bicep part, start your tricps right away. You will get a good arm pump.
My arm day was:
Biceps:
2-3 sets EZ-Curl bar curls. One set inside grip, one with the outer grip and maybe a third alternating. I do 21's no and then for a few workouts, they are painful though, but really gets the bi's pumped.
2 sets of Barbell curls. Barbell curls are real mass builders. Do heavy weights around 8 reps and less.
2 sets Standing Alternate Curls. I don't do these all the time, just when I want to change things up a bit. When you come up, rotate your hand clockwise, seems to help put more stress on the bi's.
2 sets Preacher Curls. Use the EZ-Curl bar and keep your elbows in or you will use your shoulders too much. You can widen you elbows a bit to help you complete the last rep if you aren't going to make it. I sometimes do these with dumbells instead to change things up. You can even hook up the preacher bench in front of a pulley(lat machine) and do concentration curls with the ez-curl attachment on the cable.
Concentration Curls. Sometimes I throw a couple sets in. Take a dumbell and sit on the bench. Put your elbow on the inside of your thigh and hold the dumbell palm out. Concentrate(hence the name) on contracting the bicep throught the movement and try to keep your body still.
Triceps:
I warm up doing tricep pushups(not sure of the name). Take a chair and put it about legs length away from the bench. Put your hands on the bench and feet on the chair. Dip down and come back up, like doing a dip. I put weight on my lap and do these till they burn, then I do some more!!
2-3 sets of Close Grip Benchpress. Put your hands about 12 inches or so apart on the BB. Keep your elbows in and bench the weight. Keep your mind on your triceps, its easy to let other muscles get involved in this exercise. Do fairly heavy weight, no more then 10 reps for first set. It will take time to learn the best way to do these without using too much chest or shoulders.
2 sets of Skull Crushers. Also known as forehead extensions. Take the EZ-Curl bar and lie on the bench. Keep your elbows in and lower the weight to just above your forehead and extend back up. Again, like all tricep exercises, make sure to squeeze the tri's and keep your mind focused on using the tri's only.
2 sets of Tricep Pressdowns. Use a lat machine and either the rope or V bar attachments. Stand close to the rope and keep you elbows in as you extend your arms down and complete the rep. You can change your arm position to work the different heads of the tri well(remember triceps have 3 heads, hence the name).
Dips. Some people do dips for chest, but I like to do them for triceps. By keeping your head up and body straight, you will focus on your triceps. If you lean foward and put your head down you will focus more on chest. Use a weighted dip belt and do a couple sets. Go up in weight when you can do 12 or so.
I will throw in some random exercises sometimes like dumbell kickbacks or overhead extensions, but they are more shaping exercises in my book.
As you can see, nothing magical. Just work hard and go to failure on every set. Careful not to overtrain. One of these workouts a week is probably about right. Remember you use tri's on bench day and biceps on back day also, so don't overdo it.