If a certain ad has appeared time after time, you can bet it's a winner. Write for the details of some of the offers similar to the one you have. You might even purchase a sample product or two so you can get a good feeling for what is being offered, how the ads run, and what kind of prices are successful. If you have a specific product or information that is appropriate to a specialized type of magazine, check out all the magazines of that topic. A magazine with a higher circulation may cost more to place a classified, but bargain rate publication won't be the best investment if they don't bring the inquiries.
Many products have special seasons, and classified ads are no different.
December generally returns varied responses because of the holiday season, and is not a good month to include in your testing. The summer months are generally slower for classified responses and mail order packages. The best time to try your ads are the fall, winter and spring months. The most important element in mail order advertising is to test. Not only do you need to find out if your product will sell, but you have to find out what the best price is.