-Don't have the attitude that you will get a mediocre teacher at the beginning and then search for a better one later on. Get the best you can find/afford right at the beginning.
-Make the quality of instruction the first priority when searching for a teacher not convenience, location, schedule, price, etc.
-At the beginning it is more important to have a teacher who can teach you the instrument properly than one who specializes in the style of music you prefer.
-Don't be afraid to try several teachers before deciding which one to study with.
-Individual private lessons are much better than group lessons.
-Ask the guitar teacher to play you something at the first lesson so you can see what his playing level is.
-While the opinion of amateur/beginner/intermediate players should not be completely discounted when getting teacher recommendations, at the very least some advanced/professional players should also be consulted.
-Do not accept a negative teacher. If he blames you for your lack of success or says you do not have any talent this is a sign both of a poor teacher and someone with psychological issues. You are taking lessons to feel good about yourself and to enjoy music not to have someone make you feel like a failure. The number of guitar students who can not learn to play the guitar at a decent level if they practice adequately is very small. The number of teachers who can not teach an average student to play the guitar at a decent level if they practice adequately is much larger.
-To balance out the above statement, also try and look at yourself objectively. While some teachers may blame student unjustly many students also blame teachers unjustly for their lack of success. Before you blame the teacher ask yourself if you are putting in the required practice time and are trying to follow his instructions or merely doing whatever you want.
-Be patient and reasonable in your expectations. I often get students who want or expect to play extremely difficult pieces after 6 months. You have to walk before you can run and doing justice both technically and musically to the top level of pieces takes a significant period of training.
-In the end you are the final arbiter of who a good teacher is for you. If a certain teacher's style, method or knowledge is not what you expect do not be afraid to try someone else to see if they are better.