The main purpose of copywriting is to sell or execute a call-to-action. Persuasive writing will include power words like; free, new, instant and you.
Provide useful info
Ensure you do adequate research around your topic of choice. Your audience wants to receive some sort of value from reading your copy. You need to provide them with some important, astounding or entertaining content.
You are writing the copy, so let it be you that is heard. Don't try and be someone you are not, let your personality shine through in the words and the phrases you use. If you are normally a little blunt, stick with that and tell your customers to hurry up and order now or they will simply miss out.