Read Strunk and White, The Elements of Style, Macmillan Company, New York. This gem is also available in some software packages.
Use spellcheck or a dictionary.
Here are a number of points that I noticed in the preliminary drafts of the LAS 100 essays, Fall 1996. Most of the difficulties are explained more clearly in Strunk and White.
Formality: These are written papers, not speech. One must write in complete sentences and avoid expressions such as "you see".
Organization: Each paper should be titled. The first paragraph should outline the main themes of the paper. Examine your paper to see if the succeeding paragraphs carry out these themes. Now is a good time to write a short outline of your current draft and see if it makes a coherent statement.
Parallel structure: Strunk and White say, "Express coordinate ideas in similar form" and cite a splendid example: the Beatitudes.
Less is More: Don't keep repeating yourself; avoid words that don't have a specific purpose.
Verbs: Use the active voice and specific verbs. Replace, "The stable is where horses are kept" by "Horses live in the stable".
Avoid "thing": Try to use words that add meaning. Replace "A computer virus is a thing that ..." by "A computer virus is a program that ...". This admonition applies as well to "something" and "anything".