I’ve seen a number of folks do this in the past couple of weeks so I thought I’d comment on it.
Whenever you tell people not to do or think something, they have to go inside their head and make a representation of it before they can negate it. Even if they decide not to do it as you recommend, they’ll still have experienced whatever it was like to make that picture, sound, feeling, etc.
Some folks will even go so far as to say that the subconscious doesn’t process negation at all. It will get you what you focus on even if you’re thinking of avoiding it. I’m not sure how we’d know that for sure either way but it makes sense.
Instead, whenever you find yourself telling people what you don’t want, stop. Ask yourself what you do want and feature that. Most of the time that will accomplish the same logical argument without causing them to make an undesirable representation.
It’s funny sometimes that people don’t consider the images they’re inadvertently putting into people’s heads. A friend was recently heating up some wax and half-jokingly said not to eat it. My response to comments like that has become, “I was just about to, I’m glad you said something.” That lets them make the representation in their head of how silly their statement was without me having to resist it directly.
This is different than taking a problem/solution approach. That works too. You’ll have to test it to know which converts better for your particular context.
And I wish whoever came up with the purple elephant example would have picked something else because I don’t represent that very well in my mind. I think I’m more likely to resist and wonder why the heck we’re talking about purple elephants. Why not “don’t think of a car” or “don’t think of pizza”?
So when you’re editing your copy, make sure to filter for negations and see if there’s something else you’d prefer your reader to be thinking about.