If you can't take it with you -- lock it up. This convenient Citizen cable lock will do the trick when your bike is out of sight.
Our keyed cable lock is mounted for you on the underside of the saddle. Find something fixed to the earth and wrap around it. The lock is perfect for keeping your bike and seatpost safe for a quick errand. Please note, cable locks are great as a short-term theft deterrent, but are far from theft-proof.