How do you speak clearly?

I have a bad speaking voice -- my sibilants ("S" sounds) come out mushy. If I record my speaking voice and play it back, even when I'm concentrating on enunciation, I sound... terrible. It's a voice that sounds geeky at best, retarded at worst. A little too high-pitched and monotone, as well. People have been telling me they can't understand what I'm saying all my life.

It's quite likely that I'll give many public presentations throughout my life, so being better at speaking might be worthwhile. I've lost my fear of public speaking (knowing the material well takes care of that) -- I'm just talking about the mechanics of speech. I want to be audible, comprehensible, and not sound like a moron.