Whenever I go to shops, banks, etc. I lock up my Bike Friday
PR outside.
Japan has a fairly low crime rate, but bicycle thefts are common, which is why by law bikes are now supposed to be registered as soon as you buy them (by the dealer, there's a $6 fee). Most bike thefts here are by people too lazy to walk home from the train station or by joyriding kids. Police often carry out random inspections of bikes and riders in the streets to make sure the bikes are not registered to another person or reported as stolen.
I use a bike lock that cost about $12. It's about 25 cm in diameter and consists of an 8 mm thick steel cable surrounded by plastic. I run the loop through the wheel and the left seat stay (away from the chain).
Unless somebody walks around with a pretty heavy duty wire cutter they're unlikely to cycle away with my bike, at the same time the lock is not too bulky or heavy.