Write in the positive

From the Security and Exchange Commission's "A Plain English Handbook":

Positive sentences are shorter and easier to understand than their negative counterparts.

For example:

before Persons other than the primary beneficiaries may not receive these dividends.

after Only the primary beneficiary may receive these dividends.

Also, your sentences will be shorter and easier to understand if you replace a negative phrase with a single word that means the same thing.

For example:

negative compound . . . . . single word
not able . . . . . . unable
not accept . . . . . . reject
not certain . . . . . . . uncertain
not unlike . . . . . . . . similar, alike
does not have . . . . . . lacks
does not include . . . . . . . excludes, omits
not many . . . . . . few
not often . . . . . rarely
not the same . . . . . different
not ... unless . . . . . only if
not ... until . . . . . . only when


In that most sacred of texts, The Elements of Style, Elementary Principle of Composition No. 15: Put statements in positive form offers some elegant examples of how "going positive" can enhance one's prose.