First, decide on what kind of music you want to sing. Some teachers and coaches specialize in certain forms of music. Try to work with someone who can teach you multiple styles. Chances are if you get good at one style, this will improve your performance in others.
Second, make sure that you're aware of your own progress. When you feel that you can't learn anymore from that person, find someone else to learn from.
Next, ask friends and family if they know anyone who teaches voice. You never know who may have taken vocal lessons in the past, or might teach them now.
Go to your local high school or college and ask if there are any vocal programs you can sign up for, or if any faculty member teaches privately or publicly.
Another option is to contact the National Association of Teachers of Singing. The address is: NATS, 6406 Merrill Road, Suite B, Jacksonville, Florida 32277, USA. Also by phone: 904-744-9022; 904-744-9033, or e-mail: http://www.nats.org, [email protected]. The website has a "Find a Teacher" section you may find very useful.
You can also contact The National Association for Music Education State Affiliates, who may also be able to help you find a Vocal Coach in your local area. Their web site is: http://www.menc.org/connect/stateinf.html.
Finally, as a last resort, check your local or online yellow pages under "music instruction" or in a newspaper like BackStage, the Village Voice, and Show Business. Many coaches and teachers advertise in these frequently.