I was at Melbourne Uni the other day and noticed these caged bikes. Poor bikes I thought. On closer inspection I found that this is a security measure. As yet, there's not much security but the informative signs explain that in a few weeks the area will only be accessible to card holders. To get a card, you pay $20 per year.
Personally, I wouldn't use anything like this. I like the ease of bike parking, particularly being able to ride right to the door of wherever I'm going. That said, it's good to see the uni doing something about the rampant bike theft that takes place there.
But what do you do if you don't want to cage your bike, or more likely, there is no cage in which to cage your bike? The simple answer is get a good lock. I'd recommend a Kryptonite Mini D. But no matter how good your lock, this can still happen;
All that's required to strip a bike as above is a couple of allen keys and a matter of minutes. I'd be pretty confident that the former owner of this frame left their bike out overnight; this is right in the middle of town so you'd be very unlucky if it happened during the day.
One way this unfortunate commuter could have increased their bike's security, apart from not leaving it out over night, would be to secure the wheels with something like Pitlocks or similar. These replace your quick release skewers and require a specific key to remove. They also could have at least locked their rear wheel. Even with a smaller lock they could have done so like this and as demonstrated by Sheldon Brown;
If you happen to be out and about and don't have a lock there are a couple of things you can do. My preferred method when I have to duck into a shop is to flip my bike upside down, like this;
People sometimes look at me strangely when I do this but I think that's because they don't understand. Or maybe they think I'm a bit anal. If that's the case, I can live with it. I've heard the tale more than once when someone has been able to see their bike outside through a shop window but was unable to prevent its theft. Thieves are brash and can ride faster than you can run.
Another old trick is to "ghost shift". This involves shifting your gears a few times but not pedalling to engage the gear. This means that when a thief jumps on your bike and pedals with zeal the chain will jump. It might not stop the thief but it will slow them down. That said, it might stop them too, in spectacular fashion. You just have to remember that you've ghost shifted when you get back on.
There are also things you can do to increase your chances of getting a stolen bike back. These include getting it engraved with your license number, having a few photos of it and being aware of any unique characteristics. There's even stuff called DataDotDna which will identify a stolen bike as being yours.
Finally, check out this public service announcement. Some great advice and funny too!