This is anecdotal, but I've found it to be the best exercise both for pumping the biceps and making it feel like the "lower" bicep is working.
At the end of your biceps routine, load up an EZ curl bar with some weight you can get for about 15 reps. Standing, bring your shoulders back, retract your scapulae, and keep your elbows in line by your sides. The movement is kind of like a drag curl. Bring the weight up through the ROM tight to the body until you feel your elbows wanting to move forward, then lower the weight. For me, the ROM is only about 10 inches - just the bottom of the movement. The bar usually only makes it to my belly button.
I really like the pump I get from this. I feel like it pumps the lower biceps (which it may or may not) but it definitely lends some fullness to the arms when you hit a hands-clasped most muscular. Try it out if you want and see what you think. I looked real quickly, but couldn't find the video I took this from. There are a couple of clips of Mr. O doing these - hence my inspiration
I can tell I might not be being clear enough, so if you have questions or want me to clarify just let me know.