Look for a company that has a good reputation, offers free estimates, and has competitive pricing.
Do not pay a large up-front payment. This puts you at risk for a company taking your money and not performing the service.
Make sure all promises and warranties are put in writing, including the type of paint they are using and the number of coats.
Do not cut corners on cost by using cheap paint.
Ask the company you are looking to hire what sort of prep work they do in order to get your project ready. Proper prep work will lead to a longer lasting paint job.