When writing ads, sending emails, answering questions, you need to realize that you are dealing with individuals. Communicate with these individuals just as you would if they were on the phone. You are dealing with people with all of their quirks. They have concerns, needs, insecurities and dreams. Help them to realize these dreams, avoid problems, improve their families lives, and they will be loyal, long term customers.
Nobody likes to be taken for granted.
Don't insult your customers. The biggest way I see this being done is with hype. You see ads promising the impossible. Your potential customers are intelligent and should be questioning everything they read. Don't insult their intelligence by making exaggerated claims.