I don't think biceps are the hardest muscle to train - I find that bicep workouts are the hardest for people to execute properly.
Pick any gym, any day and you'll see someone attempting the bicep curl - and horribly butchering it. Picking weight about ten times heavier than you can handle, swinging the weight up way too fast while rocking back from the toe to the heel, shifting the weight onto the legs, back, etc. pretty much everywhere but the bicep yields pretty piss-poor results. I see everyone walking right up to the bench-press station and curling the barbell as if they're the next Ronnie Coleman (when they could just be using a curl or straight bar instead of occupying the bench and pissing everyone off) and I almost just want to walk by and kick them right in the...
The preacher curl is another commonly butchered workout you see at the gym - with some fool putting way, way too much weight on the bar, sitting back way too low, grunting and slamming the bar down as loudly as possible, etc.
Rant aside, practice good, form with a slow but steady and I also agree with compound workouts - pull-ups are a great workout for the bicep when accompanied with curls or hammer curls.