Knowledgable, courteous staff that really enjoy people and not just pretend so that they can have a job. Lots of samples with kits already made up in the same colorway. Also lots of kits for small gifty item that can be made up quickly.When buying fabric, purchase enough of a line ( a complete line is wonderful) so that a complete quilt can be made from that same line.
When possilbe listen to your customers. Stock what the majority of them like and not just your personal likes.
A shop does not have to be big. A small cozy shop with wonderful staff and great fabrics can be the most inviting place in the world for a lonely quilter.
Also don't forget the classes...everyone loves classes.