For many years, a substantial percentage of my practice has been with clients who had been dissatisfied clients of other lawyers. If these same lawyers ask me how to avoid this, I often try to change the topic. I do this because the real problem is something their former lawyers perhaps cannot do anything about.
In any field of human endeavor: being operated on by a surgeon or judging the pilot of the airplane in which you are a passenger, or the person who shines your shoes, or fixes your watch, the one thing you should look for is experience.
The other qualities are intelligence, insight into people including judges or understanding human nature or wisdom or integrity. None of these are teachable or learnable.
Some few things are learnable and/or correctable: courtesy, politeness, overpromising, knowing the law and laziness.
But experience and wisdom unfortunately cannot be taught in law school. Best to keep your eyes on those indispensable qualities in law … and life.