TODO: Turn this into a FAQ
Nothing inhibits frequent use of bikes for everyday purposes than the worry that your bike might get stolen. And most stolen bikes are never recovered by their owners, even in the likely case that they wind up at a police station.
Another resource is the National Bike Registry to increase the possibility of getting it back.
Using Your Lock Securely
If you don't lock your bike, Don't Leave It!
From Bicycle Source
Most bicycles are stolen from home, meaning the yard, garage, or dormitory that the bike spends most of its time. If your bike is unattended, lock it. The best place to store your lock is on your bike!
"Bike theft isn't usually premeditated. The bike's there, it's unprotected and people take it. People need to stop providing the opportunity," says Lt. Dave Hiller, of the Grosse Pointe Park Police Department.
"Bike theft is a crime of opportunity," he said. "If you don't lock up the bike, then go into the library or somewhere else for a minute, you'll come out to find out your bike's gone. Or if you leave the bike laying outside overnight and somebody sees it at one in the morning, it's gone."
Rob Dehoff, owner of Dinkytown Bike and Skate, says "More than half the time people left their bikes unattended for just a minute. They walked away, and just a minute later it was gone. That's the key -- always lock your bike no matter what."
To avoid forgetting your lock or carrying 4.5 lb lock around on you 19 lb racing bike, try locking it to the bike rack at work, and just bring the key. Then carry a lightweight cable lock with you, used in combination with the U lock at work, for use at home or in front of the corner store, where thefts are mostly crimes of opportunity and anything will do.
Pick Your Post
Only lock your bike to a seriously immovable object. Trees, wooden or aluminum posts, and chain-link fences can be easily broken or cut, allowing the bike to be lifted over the post and carried away. Objects which can be removed, such as another bike or an unsecured bike rack, offer little protection. Look for something embedded in the concrete, such as a permanent bike rack or a parking meter, but be sure that the locked bike cannot be slipped over the top.
Pick a well-lit busy area with lots of bystanders. Don't put it somewhere secluded. Try to find somewhere supervised, such as in front of a parking toll booth or inside a building near the cashier. If you are a customer, a restaurant or shop will seldom be anything but cheerful and helpful if you ask them to watch your bike as it leans against an interior wall.
Don't park your bike so that it obstructs handicapped zones, stairs, entrances, ramps, pathways, door-opening buttons, or traffic thoroughfares. A bicycles that is found parked in these areas may be impounded, so be courteous and use bicycle racks whenever possible.
Safety in Numbers
Try to lock your bike where there are others. Thieves are less likely to choose your bike, and beyond that chances are that someone else's bike will be less carefully secured then your own. Thieves go for the easiest targets.
In the same vein, if you regularly park your bike in the same spot, look at the competition. Are they more expensive than your own? Are they less carefully and thoughtfully secured? Is the quality or type of lock inferior? When a bear attacks a group of campers, you don't have to outrun the bear -- you just have to outrun the guy next to you. If the bikes are all sensibly secured using a quality U lock, then use both a U lock and a cord-type lock or two U locks.
Lock the Frame
The best way to use a U lock is to put the U around a seriously immovable object, push your bike up to the post such that the bars pass inside both the frame and rear wheel, and then place the front wheel and locking bar in place.
Do not lock the bike to itself, such as by locking the front wheel to the frame. A thief can simply carry the bike away. Don't pass the lock only through the front wheel, rear wheel, and post. You will return to find two wheels without a frame. You must pass a lock through every component of your bike that you wish to keep, so get both wheels and your frame at once.
Fill the U
The favorite way to remove a U lock is to jamb a car jack into the U, and break it open. To make this as difficult as possible, fill the space inside the U with as many components as possible, so that a jack will not fit inside. Use your bike, front wheel, and the biggest post object that you can still close the lock around. Use the smallest lock that will still work. Some U locks are wider than necessary, making it harder to fill and far easier to fit a jack into than with a skinnier U.
Position the Lock
This is an easy technique which makes access to the lock more difficult and visible. Position the lock as high above the ground as possible, to make inconspicuous tampering and access more difficult for thieves.
If your U lock has the keyhole at one end of the cross bar, point the keyhole downwards. Locks with a keyhole on the end can be easily defeated by fitting a pipe over the lock mechanism and bending the cross bar, but this is only really practical if the keyhole end is accessible. This also keeps rain out and makes picking more difficult, without inconvenience to you.
If your lock rests against the ground or something solid, a thief can use a hammer or rock to smash the lock mechanism.
If you already have a cable lock, wrap the cable tightly around your bicycle and a fixed object, keeping the locking device as high above the ground as possible. This will make it difficult to gain leverage by bracing one leg of a bolt cutter against the ground.
Get a Quality Lock
There is no way to arrange a dog-collar-chain lock to be secure, regardless of how well you absorb the tips in this article. Get a quality lock, such as Kryptonite's Evolution series. Ask at a bike store, and check for theft-prevention guarantees, key replacement, warranty, and a track record.
A good alternative for wheel quick releases is Cyclox, a $30 locking replacement that still quick releases as long as you have the key and special tool that comes with it.
Anyone with an Allen wrench can steal major components of your bike very quickly. Expensive bar ends can be secured by filling the Allen recesses with silicone caulk, or even epoxy. It can be peeled out or chipped out, but it will slow them down and make them move on to another bike. Nice pedals can be secured by wrapping electrical or duct tape around the axle where the wrench would go. Peeling of several layers of tape takes time and the crook might move on.
Run your helmet strap through the lock when leaving your bike. There is no resale value for used helmets with a strap which has been cut. Secure your seat by bringing it with you or using a cable lock in combination with your U lock. Many, many quick-release seats are stolen, especially for expensive seats.
Repainting your bike to look old or otherwise defacing it will not deter the sophisticated thief who is able to penetrate your lock. A defaced bike, of course, is less likely to be the victim of a crime of opportunity if you leave it in front of a store while picking up a Gatorade, but then you wouldn't do that, would you?