I find that most people with poor arm development are simply using way too much weight and are 'swinging' the exercises rather then contracting with the bicep muscle. Keep your elbows tucked into your body. Your elbows should remain locked throughout the entire movement. If you elbows are swinging, your deltoids are doing the movement and not your biceps/triceps.
There are two different tricep heads and you need to do both over head and press down exercises to properly train both.There are two bicep heads in your bicep. The long bicep head is the outer bicep head and the short bicep head is the inner bicep head (the head that is closest to your chest). Both biceps heads are used to contract the biceps whenever you do a bicep exercise but you can prioritize which bicep head does the majority of the work by changing your grip width.
A wide grip (wider then shoulder width) prioritizes the inner (short) bicep heap.
A narrow grip (narrower then shoulder width) privatizes the outer (long) bicep head.
Try training biceps and triceps twice per week. Add in 2-3 exercises for biceps after your back workout, and 2-3 exercises for triceps after your chest workout. Then later in the week perform an arm workout where you superset biceps and triceps.