Reference #65: The Elements of Style
Avoid non-committal language in writing; prefer to use words in their positive form rather than using "not". Readers want to be told what is, not what is not.
For example, replace "not honest" with "dishonest", "did not remember" with "forgot", and the sentence "he was not very often on time" with "he was usually late".
Note that negatives other than "not" are usually strong, such as "never", and so may be used.
Strunk, White. The Elements of Style, 2000. (19-20)RSS