Well, I spent 2 days tweaking sequences in FCP and STP and I think I'm finally on to something. (First, I was having problems just sending sequences to STP, but then I designated STP as the external editor for video and audio files in the preferences settings and everything worked fine after that.)
My audio is definatly sounding better (even our account managers can tell the difference, and that's saying something!) I suppose, though, with the EQ and commpressor settings I am using, the audio is now sounding a little "electronic," or "processed." It definatly punches through, but I don't think I've got it really dialed in yet.
Any final thoughts?

Jay Rose has a couple of good books, at least, that take one through the process and they come with example CDs that are handy if you are learning. They allow you to hear what a good mix should sound like through your system -- and that's 1/2 the battle.

Audio Postproduction for Digital Video (1999)

Producing Great Sound for Digital Video (2002)