Improving your speaking voice

[StevieG]

I need to address a colleague's public speaking. We both know it needs improvement, but I don't want to say "we need to do something about this" without being able to say what that something is.

The main points are diction and projection (in other words, mumbling).

A simple Amazon search finds titles with books and CDs like "Speaking Clearly," but I do not want to waste anyone's time with something that might not work.

I would be interested in hearing what other people have done to address this, and hearing other people's stories about improving their own public speaking.