There are several web-sites devotes to improving one's voice: http://www.google.com/search?q=improve+ ... =firefox-a
google is a good start. Also, there is an app, I think it's called "perfect pitch", that might do what your looking for. It is a pro level app, and costs $$$, though.. "cough cough *orren*"
This may sound kinda silly, but if your still in school and working odd jobs try getting on with a good, reputable telemarketing company. I know, lame work, but I once trained CSR's. We spent an entire day going over the basics sounding more confident when speaking over the phone. If you can find one that has these basic training sessions just stay around for the speech part then jet (it's even paid voice training
)Hope that helps a little bit.