Eye-grabbing headlines. Brief, punchy sentences.
Copywriting 101 is about producing succinct, concise copy. When I was at uni we would take a verbose sentence and pare it back to its bare bones while still conveying the same meaning.
I'll keep this brief (which is difficult for me).
The 5 keys to quality copy are:
1. Use the active voice whenever practicable.
2. Delete repeated words that are repeated.
3. Be clear. Avoid the verbose. Delete unnecessary words.
4. Keep sentences short.
5. Edit. Edit. Edit.
6. Proof read your work.
I love this post titled How to Write Good (so much so it's printed and tacked my desk wall).
What's your golden rule for copywriting?
[PS - If you're interested in the translation of the Latin that appears at the top of that post (to save you Googling): caveat emptor means buyer beware, carpe diem means seize the day, et tu brute is Caesar's famous quote "and you, Brutus"; the remainder is pig latin.]