Do you live in a high-theft area? If you're in an urban area where bike thefts are more common, investing in a top-notch lock is a must. But if you live in a suburban or rural area where crime is rare, you have more leeway to balance factors like ease of use and portability with security. However, keep in mind that you'll probably still want to opt for a lock that offers a higher level of security than a cheap cable lock that can be easily cut if you leave your bike for long periods.
Will you need multiple locks to keep your bike safe? Most U-locks will fit around your bike's frame and rear wheel, but your bike seat and front wheel are still easy to access. Consider exchanging your quick-release bike seat post for one that locks, and adding a secondary lock to protect the front wheel. For instance, consider two U-locks, or a U-lock and a chain lock.
Do you know how to properly use a bike lock? Experts caution that while no lock is guaranteed to stop theft, an improperly used bike lock is almost as bad as forgoing a lock completely. Buy the smallest lock that will fit your bike properly, meaning it can secure both the bike frame and rear wheel without leaving much excess space. A proper fit means it won't give thieves a lot of room for leverage while attempting to cut or pry a lock. Also, never lock a bike by securing just the wheel -- thieves can simply pop off the wheel and take the rest of your bike.
Stick to busy areas when you're locking your bike. Buying a good bike lock is only half the battle. If possible, lock your bike in high foot-traffic areas instead of little-traveled alleys or garages. Thieves will be less willing to spend a lot of time trying to defeat a lock when passersby can watch their handiwork.
You can control just about everything with your smartphone now, and bike locks are no exception. Skylock (Est. $179) is a U-lock that you unlock with your phone. An accelerometer inside the lock can detect crashes and notify friends or family, and the lock can detect theft attempts and send you an alert. Bitlock (Est. $129) has a built-in activity tracker and a backup combination lock in case your phone is lost or out of battery. LINKA (Est. $129) is hard mounted to your bike, and automatically unlocks as you approach. It also has a built-in siren that goes off when it detects a theft attempt.