You should spend about 10 percent of the total cost of your bike on security. Generally speaking, a lock that costs less than $75 can be breached with a basic set of tools.
The Hitching Post
Trees, scaffolding, and chain-link fences can be easily cut, broken, or disassembled. Look for something embedded in concrete, like a parking meter or street-lamp. (Just be sure the locked bike can’t be slipped over the top.)
Safety in Numbers
More bikes locked up in the same spot means more people coming and going, dissuading would-be thieves. But a bunch of poorly locked bikes also makes for easy prey. Your goal should be a more securely locked bike than the next guy’s.
Divide and Conquer
Remove your front wheel and lock it to your rear wheel and the frame. In high-crime areas, use a cable lock or a length of -bicycle chain to secure the seat—or remove it and take it with you.
Fill the U
When using a U-lock, try to fit as many components (wheels, frame) inside the U as possible, reducing the amount of space a thief could slip a crowbar into. For extra security, thread a cable lock tightly through everything.