Rapport is extremely important in a business to consumer relationship. Showing that you share a common interest and/or mutual feeling of friendliness helps the relationship reach another plateau.
Keep to your commitments. For example if you tell a customer you will do something, do it! Trust helps build rapport. If you make promises that you can’t keep, you’re only making the initial build of rapport harder on yourself.
Mirror the style of your customer. Look at the way they interact and match that style. Some customers are extremely energetic via email and person, so it’s always good to show you’re the same. Don’t be overly friendly with customers who are blunt and straight to the point.