EQ or equalization can be a powerful tool to help you make sure your voice recordings sound like they should. This is especially true if you do production as well as voiceovers, of course; but sometimes the only way to get a truly clean recording is to use a little EQ even on a dry voice track.
This site provides some really useful information about the effects of equalization at specific frequencies, and this site (with thanks to my friend Stevie D for posting the link on the VO-BB) provides an overview of equalization by the octave.