Hiring a professional to tune and service your piano can be a daunting task. Let's face it, piano tuners aren't as common as mechanics or electricians, but they can command the same fee's and sometimes higher. So where do you start? For most people these days, it's right here, on the internet!
This doesn't mean you have to find your technician on the internet, but the web is a great source for pointers and reviews for a technician you might be considering. Firstly, your paino tuner should be accredited in some way. There is a national program in the U.S. called the Piano Technicians Guild. This group offers professionals the chance to get certified, in which they'll have to pass rigorous tests that will prove their skill to you, the piano owner.
A school in Boston called the North Bennet Street School offers a program to train piano technicians in the field of piano technology. A student of this program has to pass tests before they receive their certificate and graduate.
Either of these certifications are sufficient, but I would stay away from hiring someone with neither of these affiliations. Look at www.ptg.org to learn more.
Ask your piano teacher who they use or recommend. I'd say about 90% of tuners hired would report they were recommended by another client. Referrals go a long way in the piano industry, and once you find a good tech, you will probably keep them for life and recommend them to all your friends and neighbors.
Online, there is Angie's list and Yelp. These give you a strangers reviews though, and so aren't as strong as a friend or neighbors feedback.
Your local piano store is another great way to find a good, local technician. If a tuner has to drive for more than 50 miles, they will likely charge you more than someone around the corner.
When you have hired a tech, don't be affraid to ask them to play a bit when they are finished. The piano should sound beautiful, and you should be satisfied. But if you aren't, let them know there might be a problem and see if they can get it resolved before you move on to another tuner. They want your business, and it's not uncommon for someone to come back out and resolve some tuning problems at no extra cost to you.