Whether you are keeping your bike at your workplace, just running in for a quick trip to a store, or leaving your bike overnight in your backyard, using a bike U lock is one of the best ways to secure your bicycle from would-be thieves. They can be used alone as a total security solution, or in conjunction with a chain lock to secure all pieces of a bicycle.
A bike U lock is very tough, but is not totally unbreakable. However, a thief is going to need some serious power tools to break through a quality modern U lock like those offered by Kryptonite or OnGuard. Proper lock placement is critical to getting the most security possible from a single lock. It is often recommended to place the lock around not only the bike frame, but also one of the wheels, as a thief is not nearly as likely to steal a single wheel as he is both. Wheels fetch a far higher price when sold in matching pairs.
The rear wheel is typically more expensive to replace, but depending on the U lock used and the design of the bike, locking both the frame and rear wheel may be impossible. In such cases, locking the front wheel instead is a proper solution that should deter the majority of thieves.
In addition to locking both the frame and a wheel, a U lock of the proper size should be chosen to get the most secure fit. One way that thieves may try to remove your U lock is with some sort of leverage, and they may try to pry the lock apart. Keeping this in mind, try to pick a lock that will just barely fit over the frame and wheel without much open space to prevent such attacks of leverage from occurring.
A chain or cable lock can be used alone or together in unison with a U lock. Adding a second lock like this will make your bike significantly more secure, because the tools used to hack open a U lock are not going to work against a cable lock. This means that a thief must be carrying around two separate tool kits in order to even begin stealing your bike, and this is something that is not likely to happen in practice. They are much more likely to be carrying around a single set, and will simply pass by your bike for one that is easier to lift.
When using a cable lock to supplement a U lock, it should be placed around the less important part of the bike because they are more vulnerable to attacks than a standard U lock. While they will prevent a casual thief from picking up your bike, a more seasoned pilferer will know how to snap through a cable very quickly and easily.
Bike security is important not only to save money on a potential replacement bike, but also to save the time and inconvenience it will take to get home when you have no transportation available. Nothing is worse than coming out after a hard day at work only to find you have no way to get home. Having and properly using a good quality U lock will prevent the majority of attacks from happening, and make sure you have your transportation when and where you need it.