I have On-Guard D lock with key, with a 5' long cable to tie my bike on my rack if I'm going to be inside a store or put it in my car, or if someone wants to lock their bike on my bike rack as well. They have a guarantee that it won't be cut through, and if it does, they will replace the cost of the bike/frame (I think, please double check on this...).

I use the D lock on my rear wheel to the frame, and if I'm using the cable, I wrap it around the bike rack.

I never EVER leave my bike alone overnight tied to something (with exceptions of Triathlons where you need to leave your bike overnight - security is always there).