I bought a CD from Liz Von Seggan of OneWay Street in 2003 from which I got the basics. From there it was just "practice, practice, practice!" The fun thing about "venting" is you can practice while alone in the car and nobody will know! (ha)
Once you learn the ventriloquist alphabet, it's just a lot of practice. It also helps to attempt to stick to a script. Preschoolers are an easy group to practice with as well. They really don't care if you move your lips (ha!)
Good luck!
Sparky
LAFS For Life
ACTS Entertainment
ACTSClownMinistry.org