If you have great copy for your ads, you are most likely doing a respectable amount of business on the web.
All great copy has certain things in common.
First, and foremost it has no spelling or grammatical errors. Nothing turns a prospect off faster than these types of mistakes. In addition to running all your work through a good spelling and grammar checker, having someone else carefully read what you have written is also a good idea. Use short ads of two or three sentences with meaningful headings. Most people won't read lengthy online ads.