I'm paranoid about having my bike stolen. There are a couple of places (high foot traffic) that I will leave it for a short time with a cable lock (e.g. the farmer's market), but for most stores I use a mini U-lock around the rear wheel inside the rear triangle locked to a rack and then lock the front wheel and frame to the rack with a cable lock. A regular U-lock would go around some railings that the mini might not, but they won't fit around poles, etc). Theory is that most thieves don't carry both kinds of tools (or at least it would be a major pain in the you know what to steal). Not sure about mounting - I usually throw the locks in a rear pannier (which you may want anyway if you are running many errands and will need to carry things).
If a store doesn't have an appropriate rack, the bike comes inside with me.
CA
Most days in life don't stand out, But life's about those days that will...