Start by trying to get a referral to a patent attorney from someone you know such as someone who is an inventor, entrepreneur or startup. Alternatively, ask an attorney you know for a referral. Getting a personal reference helps get the attention of the patent attorney you are contacting and makes it more likely they will do a good job because of accountability to the person who referred them.
Alternatively, try a website such as AVVO.com or the attorney/agent search at the USPTO.
Also, depending on where you live, consider working with a patent attorney that is not local. Since patents are federal law, patent attorneys can work with clients in any state (or event worldwide). For example, you might find a more qualified patent attorney candidate in a larger metropolitan market, or you might find more affordable options away from a larger metropolitan market.
Check out the following for links to the websites above and a helpful patent attorney screening form to help sort candidates. http://www.patentsdemystified.com/chapter-8-picking-a-patent-attorney/
Additionally, Chapter 8 of Patents Demystified discusses the process of finding and choosing the right patent attorney if you want more detail.