To simplify text so that all readers can understand, write at a level aimed at sixth grade. The Flesch Readability Scale (found on newer versions of Microsoft Word and WordPerfect) and Gunning's Fog Index are two well-known tests of readability.
For more on these tools and others related to writing for readability, there is an excellent publication by Debby Weitzel, Publications and Promotions Specialist, Colorado State University Extension. The publication can be found at: www.ext.colostate.edu/PUBS.