Where are you locking up? In a big city or out in the suburbs. In the city, the Target/Walmart lock are very easy to brake. I use an ABUS U-Lock. But it's not just about locks and WHERE you lock up is just as important.

Check out Hal:


Three local lessons from Minneapolis:
My daughter left her SS commuter outside overnight in front of her apartment, front wheel and frame locked with U-lock, rear wheel stolen.
My daughter college roommate, used a very nice U-lock through the front wheel. She was left with a front wheel and U-lock but no bike.
My friend used a good U-lock but the post he lock up to was not securely anchored. They pulled the post out of the ground and the bike, with lock, was gone.

So, get a good quality U-lock and then do a good job of locking the bike.