If you have great copy for your ads, you are most likely doing a respectable amount of business on 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. In longer ads, such as on a web site, use of bulleted lists, which gives an overview of key points is effective, and will make copy more readable.
If you are advertising on a web site, there are other conventions you should follow. If it is an ad, it should also be short, and follow rules above. If however, someone who has read your ad, is coming to your web site for more information, it should not leave person reading it with questions that are not answered there.
Never, and that is a very long time, use images that take too long to load. If you do, odds are you have just lost a customer. The text should be easy on eyes and forget cute background and text combinations. Dark blue on a black background is simply too difficult to read.
A white background with black text is still best choice, however dark text on a light background is also acceptable. Using an image for a background is a mistake, and will take away from effectiveness of your presentation.
All caps, either in an ezine ad or on a web page, should be avoided. They are very difficult to read and most people won't. Also, forget fancy fonts. Arial and Times Roman are both very legible onscreen and if printed give a high quality copy. A fixed-width font such as Courier is preferred font for email ads.
While many publications use indents to start each paragraph, a blank line separating them is friendlier to reader. Their use quickly lets reader jump from one to another. It is also a good idea to have paragraph headings, which lead in to next paragraph. Many people scan these headings, and if they don't get their attention, they skip to next. Obviously then, paragraph headings are extremely important.