The only way not to have a bike stolen is to have your butt or your hand on it at all times. Even that doesn't work sometimes.

I use a D Lock lock and a Hawser. Weighs a ton so dont take it on rides often. Hawser through the wheels- frame and seat rails then round an immovable object before being locked with the D Lock that also attaches the Frame to the immovable object.

But if a thief wants your bike- it is going.