The explanation for this one is simple, but it's far from easy. Do a close-grip chin-up as slowly as you can. Shoot for 20-30 seconds on the concentric (lifting) and 30-40 seconds on the eccentric (lowering). If you can do that, add some weight. If that's too much, then just go as slowly as you can. Add a static hold at the top if you're a bad ass.
The One-Rep Chin-Up
Use a supinated grip (palms facing you) for more biceps emphasis or a neutral grip (palms facing each other) to target the brachialis. Either way, it'll also blast your forearms and help build tremendous grip strength.
Save this for the tail end of your workout and just do one painstaking rep. If you're doing it right, that's all you'll be able to muster.