Voice One is a voiceover school in San Francisco. I took classes there for about a year/year and half fairly steadily, and continue to pick up classes when there's something that interests me. Through classes at Voice One, I met a director who gave me a voiceover job, I solidified a relationship w/a local casting director who has called me in for a few projects, and gotten me a few jobs, and that's how I met my agent. And, I learned about voiceover, made demos, and took a few on-camera classes as well.
In general, voiceover classes aren't cheap, and take a lot of your time. But if you want to know more about Voice One, here's where to go.