The best advice I can give for your email marketing campaign is that they must keep your newsletters entertaining, understandable and fun. Always remember to include your website (if you have one) in your newsletters, that way if your readers want to know more about your company, they can click and go to your website.
If you really want to keep your customers continually buying yourr products / services, create a strategy, such as:
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You can also make your current customers recommend new customers and also give prizes / discounts for each customer they refer (Like a referral program). This is also a good strategy to grow your database.
So it's really simple to keep your customers happy, and if they are happy, that will benefit you in all ways.
So keep your newsletter updated, easy to understand, entertaining and make good deals so your clients can take advantage of them and purchase from you!