Jonathan has published an article about bicycle theft prevention over at BikePortland. All of the advice is what I consider common sense, but bikes keep getting stolen so I guess not everybody knows how to lock up their bikes. Among the theft prevention tips:
Use a good lock. Thin cable locks are fine if your bike is in your plain view, but U-locks are the only good deterrant.
Don't keep the bike on the front porch. I've known several people who've had their bikes stolen from the front porch, even when the bike is locked to something.
Don't hide the bike. Don't park your bike back by the garbage dumpster or behind a wall; that allows the thief to work without observation. Put the bike out front where there are people.
Don't just lock the front wheel. To me it's unbelievable that people do this, but I see it all the time. I also see a lone wheel without the bike locked to a bike rack all the time.
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I had a friend in DC whose Fuji was stolen back in the '70s after he Kryptonited it to a fence by the front wheel. He said the moment he saw the wheel locked to the fence, and no bike, he realized his error and walked right past to the bus stop. He was too embarrassed to even retrieve the lock.
I noticed on the Portland Stolen Bike Boards that a bunch were taken from in front of a 24-Hour Fitness place. Have they considered doing a sting operation? The people stealing these bikes are probably repeat offenders.