I hesitate to send so obviously self-serving a message, but Mary Epes
and I have just finished a workbook (aimed at college students of every
level) which addresses just such questions, and provides information and
exercises which have in my experience been able to help students improve
both the correctness and effectiveness of their sentences.  It is called
Mastering Written Sentences and will be published in December by
Prentice Hall.  It might be of interest to some of you.
 
Michael G. Southwell =================================
DNEBA Enterprises                     Department of English
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Bloomingdale, NJ 07403-1425               Jamaica, NY 11451
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