In my opinion, pretty much everything that everyone above me has said is true. I don't think it's really hard to work your triceps. I mean, you're already getting a lot of triceps work when you do any pressing movement. Any time you straighten your arm against resistance, your triceps are going to become involved. Now, as far as getting your triceps to get bigger, that just comes with time and overall size. If you want larger triceps, try to gain overall mass. Just know that your arms are only going to get so big before you need to increase your overall size.
Also, sometimes less is more. I've found out that for me, doing less direct arm work actually makes them respond better. I like to tack on a little direct triceps work at the end of a chest or shoulder routine. Actually, I'm starting to think that I really respond well to working my body in a push/pull/legs fashion. Or in other words, if I'm already working my chest and shoulders, my triceps respond well to being directly worked at the end of that routine. BUT I can't do high volume for arms or I'll be overtraining them. Here's an example of a couple push workouts that I've done...
Chest/Shoulders/Triceps
Incline DB Press 4 x 10, 8, 6, 4-6
Flat DB Press 4 x 10, 8, 6, 4-6
Arnold Presses superset with Rear Laterals 3 each x 6-8
x 1 iso triceps exercise (usually extensions, sometimes pushdowns) 4 x 6-8
Shoulders/Chest/Triceps
DB Shoulder Press or Upright Rows 4 x 10, 8, 6, 4-6
Side Laterals 4 x 10, 6-8
Incline Bench Press 4 x 10, 8, 6, 4-6
Superset Incline Flyes with DB Pullovers 3 each x 6-8 (pullovers x 12-15)
x 1 compound triceps exercise (GG curl bar bench or dips)
So I try to keep in mind how much my triceps are already working during my chest/shoulders work, but since they've already been worked and warmed up, I'll go ahead and work them directly AFTER my chest/shoulders work.
For me, less is usually more. Now, I love working my arms and if I could, I'd work them all the time, but I'd be counter productive. Try working them less and see if you notice them becoming stronger and fuller.