Before you write the first word...RESEARCH, RESEARCH, RESEARCH! If you don't do adequate research, you cannot write an adequate ad! Up to 80% of a copywriters time on a project, should be for research. If you do not know your product, how can you convince someone else to buy it?

Positioning is the next item after research. Why should someone buy your product? Just because you say it's better than so and so? BULL! People are so tired of hearing about how this product is better than another, because I say so. You need to offer PROOF! If you have followed the first item, research, you should be able to come up with a USP (unique selling point). Tell the audience why you are better than the competition.

Know who is going to be buying your product. Make sure you target the audience that you are trying to market to. You need to know the demographics of your audience, for the media you are using to advertise. It doesn't matter whether you intend to do direct mail, internet marketing, space ads, etc. There are subtle differences between online and offline marketing and how you approach each. You need to know what they buy and why they buy, before you can successfully sell to them. You need to put yourself in their place and see if the product would appeal to you.

Plan how your copy will be used. How much copy is enough? That depends upon the audience, the product and the medium you use to convey your message. Usually, longer is better, but not if it looks like an advertisement. It has been proven time and time again that an editorial style ad will get read alot quicker than something that looks like an advertisement. People are tired of being sold to. You have to use finesse and give them information that they are looking for and then use this information to lead them to your product. If you have a simple product that research has shown you people are looking for, then you might not need as much copy. Again, the length of copy can vary according to the medium you are using to advertise in.