Your biceps bend your arms, and you work them with curls. You probably figured this out as a kid when Dad told you to "make a muscle" for one of those photos parents use as blackmail for when their sons start dating. Chances are you did a curl the first time you ever picked up a dumbbell, and you've been doing them ever since. Now ask yourself: When's the last time I increased the weight I use on my arm exercises?
Physiology 201: If you want your arms to grow, you need to create overload and challenge them with progressively heavier weights. They'll adapt by growing bigger and stronger. Since they aren't receiving that overload from curls, you need to recruit bigger muscles to help them grow in tandem.
Start with the chinup. This move forces you to lift your entire body—several times the amount of weight you could curl—on each repetition. Your lats, which are the fan-shaped middle-back muscles that run from your armpits to your spine, do a good deal of the work. But your biceps are more than just bystanders. They're working as hard as they can. Without their help, you couldn't do a single rep.
Target your triceps the same way. Do body-weight dips or close-grip bench presses with a loaded barbell. You'll be able to lift multiples of the weight you use for extensions.
Make these heavy, multimuscle exercises the focus of your upper-body training. After you've performed them, add curls and extensions to give your arms some extra oomph.