My friend Aaron had his bike stolen today, a common occurance here in New York City. My bike lock cost 1/3 the price of my bike itself, but it's a necessary investment. The Kryptonite New York Fahgettaboudit lock was specifically designed to foil the most agressive bike thieves. Yes, it's pricey. Yes, it's heavy. Yes, it's a pain in the ass to shlep around and use. But yes, it's worth it. Just ask Aaron. Starting at about $75 at amazon.com.