While most thefts are crimes of opportunity, locally there are guys with trucks and lock-breaking equipment on the hunt for good bikes to steal. You can, however, reduce the risk of theft with the usual advice:
"Uglify" your bike. It's generally acknowledged that ugly, scratched non-descript bikes will attract less attention than a high-dollar boutique bike that's in polished and mint condition. Because knowledgable thieves know how to look past the ugly to the bike underneath, Cyclelicious recommends a cheap beater bike for areas with higher risk of crime.
Always lock your bike. Bike theft occurs even in small, safe towns. In some areas, cable locks may be sufficient. U-locks work well for more security. Because the methods and tools required to defeat cable and U-locks are different, many people combine the two, using both cables and U-locks.
Remove the slack. Use the smallest lock or cable needed. Extra slack in the cable makes it more convenient for the thief to snip the cable. Extra room on a U-lock is room for a thief to fit a jack to break the lock.
Keep the lock off of the ground so the thief can't just smash the lock against the ground with a hammer.
Remove accessories. If your bike is parked for any length of time, remove lights, bells, computers and anything else that's easily removable from your bike. Consider replacing a quick-release seatpost binder with a real bolt and nut -- the thin saddle-rail cable locks are worthless.
Fill bolt heads. In high crime areas, experienced cyclists melt solder into allen key bolt heads. Solder can be heated and easily wicked out when service is required on the secured part; the same is not true for other similar measures such as wax or hot glue.
Out of sight out of mind. Some experts recommend parking your bike in a highly visible area. Bystanders observing a theft will do absolutely nothing to stop the thief, however, so a hidden location may be a better choice in some circumstances. When I go to a fast food place, I park my bike in the open -- visible from the restaurant windows. If I'm in the woods hiking, I conceal my bike as best as I can or make it look like it belongs to a ranger by locking it up to a park or forest facility (with permission).
Lock the bike to a secure structure. I've seen bikes leaning on a bike rack and a cable wrapped around them that aren't actually locked to anything. Be on the lookout for cheap "dishwasher" racks with loose nuts and bolts that facilitate easy bike removal. Ensure your bike can't merely be slipped up and over the object you're locking to.
Secure it at home. I've had friends with bikes stolen right from the front porch of their homes in safe neighborhoods.
Register your bike. Many localities offer free or inexpensive bike registration. If the police find an abandoned bike, it's a simple matter to look up the serial number. Write down the serial number and keep it in a safe place along with a photo of the bike; you can give these to the police to help with recovery if your bike is stolen. The San Francisco Bicycle Coalition has a clever "freezer registration" form here (PDF file).
Please leave a comment below about additional anti-theft tips, uglifying resources, or personal anecdotes of bike theft or recovery.
IMPORTANT: Please post comments for this article at the new CYCLELICIOUS 2.0 version of this page.