Find ezines whose target market matches yours, but whose publishers are not direct competitors. Try to find ezines that are complimentary to yours, e.g. a winemaker might want to place an ad with an event organizer, or a chef.
Contact them to see if they accept classified ads, or ad swaps, where they would place an ad in your ezine in return for running your ad in theirs. Use an ad-tracking program to see how many clicks you receive, and how many subscribers and sales result from those ads. I use my shopping cart’s built-in ad-tracker module.
To find ezines that accept ads try here: New-List.com, and Directory of Ezines