Miss P gave me some good advice, which I will now give you
Maybe you should start by reminding the professor who you are, since you haven't seen him/her in years, name the classes you had with them, tell them what you've been up to and what you're doing now, why you are choosing to go to law school and then tell him why you are choosing to ask him. Maybe you feel that you did some of your best work in his class and that he can attest to your ability as a student. Tell him how much it would mean to you, but make sure he feels that it's ok for him to say no if he doesn't think he can write a strong rec for you.
But all this should be done only if you can't find a way to meet with him. Since it's been a while, the best way would probably be a face to face meeting.