Thanks everyone for heir suggestions.  I have settled for now on this:

#+LATEX_HEADER: \usepackage[letterpaper]{geometry}
#+LATEX_HEADER:
\geometry{verbose,tmargin=2.5cm,bmargin=2.5cm,lmargin=2cm,rmargin=2cm}%
#+LATEX_HEADER: \usepackage{paralist}
#+LATEX_HEADER: \let\itemize\compactitem
#+LATEX_HEADER: \let\description\compactdesc
#+LATEX_HEADER: \let\enumerate\compactenum
#+LATEX_HEADER: \usepackage[small,compact,calcwidth]{titlesec}
#+LATEX_HEADER: \titlespacing{\section}{0pt}{*1}{*0.2}
#+LATEX_HEADER: \titlespacing{\subsection}{5}{*0}{*0}
#+LATEX_HEADER: \titlespacing{\subsubsection}{10pt}{*0}{*0}
#+LATEX_HEADER: \usepackage{enumitem}

which does a pretty good job of compressing the whitespace overall.  I'd
still love to do more finetuning of the formatting but I think that's a more
complex project.  Unfortunately this seems to have broken mk4ht for me!
which is a real drag as it's the only way I can reliably produce odt
documents, which I need much ofthe time.

Does anyone know whether numbering can be turned on just in some
subheadings?  That would help me quite a bit...

thanks thanks thanks for al lthe assistance,
matt