Yeah, I second calling the dept or his secretary or whatever.
And i'd be direct, and to the point in the message, but you don't have to go in great detail, otherwise he'll get bored.
I'd say something like "hi, this is GreenEqsnHam, I graduated from School Univ in 2000, and I took your class "education101". I received an A. I am applying to law school in the fall, and was wondering if you could write a recommendation for me. I would greatly appreciate it if you could give me a call at your earliest conviencence to discuss futher. Thanks very much, and I look forward to speaking with you."
Or something along those lines: short, direct and polite.