It's easier to remove excess audio information than to re-create missing information.
It may be difficult to give a tinny sound more body, unless the problem is simply too much high end. You could try lowering the upper frequencies with parametric EQ, then rais the volume of the track to fix.
The best things to prevent these problems are to first, make sure you're using a good microphone. I've recently been reading and shopping for a good audio microphone. I've settled on a Neumann M-149. I was stunned to actually hear the vast differences between microphones! If you can borrow or rent a variety from a local supplier, prior to buying, you'll appreciate the time spent.
Next, be very careful with digital processing, particularly noise reduction. They tend to look for and remove patterned sounds like fan hum, etc. The problem is that voice is full of the same patterns, and unless you are good at it, noise removal can remove a lot of body from voice.
A good condensor mic with a large diaphragm will do wonders. Also, make sure you're bringing the audio into the Mac with proper equipment. Although an inexpensive mic directly into the Mac can work, it is profoundly lower in quality than using a USB or Firewire audio interface and a balanced mic signal.
Good luck!
