Compile your own email base and send a weekly or monthly newsletter to your readers.
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Choose an affiliate tracking method.
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Order a text or banner ad in relevant third-party mailing lists.

Think about relevant newsletters and related products providers that may offer advertising in their mails. This kind of partnership will be profitable for both of you.
Do not forget to track the results of your advertising campaigns with the web analytics solution of your choice, e.g. Google Analytics.