In my previous tip, I advised writing in the positive form and avoiding the use of “not” before an adjective. Double negatives are even worse because they make the reader think too hard about what you actually mean. Do two negatives make a positive? Sometimes. Maybe. Just avoid any confusion in the first place.
Double negative: That’s not to say she didn’t enjoy the show.
Positive: She enjoyed the show.
Remember that you can still express negative ideas and words in positive form:
Double negative: That’s not the only reason the smartwatch is not a good choice for runners.
Positive: There’s another reason the smartwatch is a bad choice for runners. (Or, just say what the reason is: The lack of GPS makes this smartwatch a bad choice for runners.)
not uncommon → common, fairly common, relatively common
not unlike → like, similar to, resembles
not infrequent → frequent, regular, common
not horrible → mediocre, good
not worse → better