If the only thing you do for your biceps are curls, you'll never build big arms. Hypertrophy isn't just a localized process. It's systemic. That's why a big healthy meal of chin-ups will do more for biceps growth than a dessert of dumbbell curls. Try this:

15 Minutes of Chin-Ups

1. Do chin-ups for 15 minutes, resting as needed. Tally up how many you were able to do. Write that number down.
2. Here's the key: Do this once per week and try to beat your previous score every week.

Notes: Vary your grip as needed during the 15 minutes. You may want to start with the standard chin-up grip: supinated or palms facing you, medium to narrow hand spacing. As you fatigue, switch to a neutral grip, palms facing one another. Most gyms have a set of pegs or handles for this. You can also toss a V-bar attachment over the pull-up bar.

If you're one of those 150-pound guys that can do 4,000 chin-ups in one set, then add enough weight so that the first set brings you down to around 15 reps.

You can still do direct arm work on another training day as dessert, but fill up on a hearty meal of chins to really feed your biceps.