Start with a classified
Actually, thinking about copy gets in the way. You start writing and then you patch and layer and write and dissemble and defend and write and the next thing you know, you've killed it.
So, try this instead:
Write a classified ad. What's the offer? What do you want me to do? You're paying by the word!
"Lose weight now. Join our gym."
Six words. Promise and offer.
Now, you can make it longer. Of course, if your gym is on the space station and it's the only gym around, and if the people reading your ad are looking at the bulletin board and seeking out what they want, then your ad is now long enough.
But most of the time, in most settings, a little longer is better. So, add a few words or even a sentence. Is it better? More effective? Gently and carefully add words until it's as effective as possible, but as short as possible.
Perhaps you want to make your promise more vivid, or more clear. Perhaps you need a testimonial or two to back up your promise. Perhaps your call to action needs to be more urgent... You can play with all of that, keeping in mind the original classified, keeping in mind that you're still paying by the word (because attention is expensive).
And yes, this applies to articles in the newspaper, to blog posts, to how-to books and to direct marketing letters. It applies to the emails you send and the copy on your website too.