Well, I know this is a basic one, but I MUST start this weekly copywriting tips with "less is more" because although it is an obvious thing that copywriters learn at first, I can see that lot of people forget about it.
Being a copywriter doesn't mean you have to write and write and write and... BOOORING.
Really, try to use more often the delete key.
Use the necessary and correct words.
Can you think in any example of a too long copy?