Whenever I see someone mention bi's and tri's together in the context that one is growing and the other isn't, the first thing I always think is that they are being trained the same way.
As it turns out, the tri's are mostly fast twitch muscle and respond best to heavy lifting. The bi's on the other hand are mostly slow twitch and respond better to high reps. As with all muscles, you need to switch things around now and then but bi's should be trained mostly with volume while tri's should be taxed to failure most workouts.
The 21's that crazyjoe mentioned are great as are the incline curls the Mac173 spoke of.
The other thing that works great with bi's is drop sets; curl with dumbells to failure, then immediately grab two dumbells 10lbs lighter and curl to one rep higher then you got on the last set. Then drop another 10lbs and go one rep higher than that...etc. The key is to take no rest breaks as you work down the dumbell rack.
You get to the end where you're grunting and struggling to curl 15 or 20lbs and you hope like hell a cute girl doesn't walk in right at that moment.