If you want to annoy a reader, use negative words. Not only are negative words annoying, but research shows that it takes the brain longer to understand a negative statement than a positive one. They cause confusion!

In writing, negatives include 'un-' words like 'unnecessary' and 'unless'; verbs with negative associations like 'avoid' and 'cease'; as well as the obvious ones like 'not', 'no', 'except', 'less than' and 'not more than'. When readers are faced with a negative, they must first imagine the positive alternative then mentally cancel it out.

A single negative is unlikely to cause problems, though many an election voter has paused confronted with the polling booth challenge:

Vote for not more than one candidate. (unclear)
Vote for one candidate only. (clear)

But when two, three or more negatives are gathered together in the same sentence, meaning may become unclear, as in this note from a lawyer to his client, an underwriter:

Underwriters are, we consider, free to form the view that James Brothers have not yet proved to their satisfaction that the short-landed bags were not discharged from the ship, and were not lost in transit between Hong Kong and Singapore, when they were not covered by this insurance policy.

The above sentence is very confusing due to the number of negative words!

Here are some examples of how to rewrite negative sentences more positively:

We will not help unless you give us a special mention.
We will help if you give us a special mention

The corporation will not pay unless employees also contribute.
The corporation will pay only if employees contribute.

Your credit will not be extended until you pay us what you owe.
We will extend you credit if you pay what you owe.
We would be happy to extend your credit once we receive payment for what you owe.

We are not open on Saturdays or Sundays.
We are open from Monday to Friday.

We were not prepared for your request.
Your request caught us by surprise.