Nancy, if the purpose is "writing better," then the two possibilities
for a focus are "correctness" and "syntactic maturity." I personally
do not believe that either can be improved through direct instruction,
but that doesn't help you in your situation. I advise you to read
Constance Weaver's latest and greatest book, GRAMMAR IN CONTEXT, in
which these issues are discussed (including the research record to
some extent), and in which you will find suggestions on how grammar
can be brought into activities designed to improve sentence writing.
        --Bill Murdick