People will give you their business to you if they know you have the expertise to do a good job for them. There are several things you can do to position yourself as an expert. See yourself as an expert . If you do not consider yourself as an expert then you have two choices become an expert fast or find a new profession. I would guess you know a lot more than you think you know.
Some industries have specific designations that signify expertise in that field.
For example a CFP designation is used to acknowledge expertise in the financial planning industry, a CLU in the insurance industry. An MA or a Ph.D. always signify expertise even if the degree is in a different field. What is considered expertise is different for each industry. If you are not sure find out if their is a professional designation in your industry. If there is -get it.