I have similar Ortlieb bags to those Doug shows in his photo. I pull the looped straps over to the centre of the rack and use a small combination padlock to connect them to the rack. Sure, a determined thief could cut the straps with a knife or good scissors, but as others have said, this stops the opportunist thief.

We place high value on the contents of our panniers because we need them, but in reality there's not a lot of value to a small tent and a used sleeping bag. I never leave my handlebar bag because it contains the actual valuables.

For the bike, I carry a small Kryptonite D-lock and a heavy cable. This adds an unfortunate amount of weight, but for me it's worth it for peace of mind when I leave my bike.

Cheers,
James