There simply is no other form of advertising that is as effective as ezine advertising.
Think about what good ezines do. They offer information, education, and entertainment to readers. They target specific audiences. They go out regularly to opt-in individuals, and they present their advertising as an important feature that readers should look over.
Think about what ezine publishers are trying to do. They continually search for additional subscribers. They do their best to keep those subscribers actually reading ezine on a continual basis. They present their advertising as best as possible to give advertisers good results in hopes of getting repeat ad sales.
No other form of advertising on Internet has these same qualities. Nothing else has regularity of ezines. Nothing else has consistent fresh content of a good ezine.
Turning a profit from ezine advertising requires an advertiser to take advantage of these qualities.
Targeting appropriate ezines is essential to your ad campaign. There are many ezines dedicated to bringing information to those who work online so often this is an easy task. However, consider your product and match it with content of an ezine before advertising with it. Since ezine publishers are targeting an audience with their content, it's easy to tag your ad along with that content if it fits your product or opportunity.
Repeat advertising is also essential. One good ad will generally get results, but not everyone that could potentially respond to your ad will do so on just one exposure. There are many factors to be considered. Not everyone will see your ad in an ezine every time, not everyone will be in mood to look into new products or opportunities every time, and not everyone will be ready to buy every time.