Take your time. Sit down in a quiet place for a few hours to get started. Do your homework and write things down. And, don't be surprised if it will take longer than you expect. Just because it looks and sounds good, doesn't mean it has meaning.
Customers can sense the emptiness of a slogan from far away.
The best you can expect from one is that people will ignore it. Know your competition well. Your customers are shopping and so should you. You need to know exactly how you stack up against other businesses offering a similar service to yours. Know your market.