We've all heard the stats about how it's more expensive to attract a customer than retain them, but how exactly is this ongoing relationship so lucrative? Obviously, trust, a great customer experience, and a quality service or product are the bedrock for retaining customers. Ideally, though, your customer's relationship with your company will expand over time, as opposed to stagnating. Deciding when to send an upsell email is only a part of the equation for reaching maximum conversions. You also have to figure out what to sell and how to present it. Here are some upsell email best practices to add to your mental toolbox.