Tell them what you have to offer and how you can help them. Remember, people often scan for information, they don't actually read very much. Be sure to use whitespace effectively. That is, make sure you make your ad easy to scan, for URLs and email addresses especially. Also, be aware that you can't use boldface or italics.
Good organization is critical.
Don't use ALL CAPS . It is bad form. And, it is hard to read. Don't make any outrageous claims. Don't trick people.