Nothing legal will stop someone who really wants it, so I use the cheap coated cable locks four-for-$10 or so at box hardware stores when my bikes are pretty much within sight. Out of sight, I also add a good U-lock. They just keep the honest people honest and, like the old joke about not having to outrun the bear, just the other people running from it, any additional time a thief estimates it would take to steal, the more likely he is to grab an easier bike (or smash the hell out of your bike out of spite, the scumbags). My other theft deterrence is having bikes that don't look appealing to thieves.