Elbow extended is when they are stretched furthest... in your warm up, try this: stand in front of a squat rack and hold your arm out straight in front of you and grab the vertical support of the rack, THUMB DOWN. slowly pivot away from the rack to get a good stretch in your bi's. That's a bi stretch, BUT usually on chest day, I do a few sets of 'activation' type work for my bi's to help stabilize the elbow joint and "tighten" things up to improve my form for benching, etc. (heavy weight, low rep... get the CNS jacked w/out fatigue).
Also make sure you're being "smooth" (AND SMOOTH IS NOT SYNONYMOUS WITH SLOW!!!) w/ the weight and not lazily "dropping" it back on the pins/rack. Move w/ a purpose, not w/ momentum!