You really have to make your customers feel like you are listening to their cares and desires.
There is no quicker way to turn someone off than to blast them with content in which they have zero interest. If you want them to take an interest in you, you have to be relevant. Personally, I enjoy simple email content that points me easily to something I have intense interest in, but there is no quicker way for me to hit that “Unsubscribe”, “Mute”, “Cancel”, or “Unfollow” button than to spam me with trash that I could care less about or even worse waste my time on the phone or in person.
In many cases you are selling more than one product and in to more than one market. Make sure that your newsletters, emailers, etc. all give your customers an option as to what content they want and what they don’t and make sure you comply with that request. That doesn’t mean you can’t make suggestions or add links here and there to something that you think they might also like (associative sell), but keep it rational if you want to keep them as a customer.
Just because someone was kind enough to share their email address with you, follow your social media feed, pick up the phone, or give you an in person meeting doesn’t mean they are giving you the right to waste their time or bombard them with irrelevant junk. Your customers put their trust in you, so before you ever hit that send button, ask yourself “Am I being relevant to those I am about to blast?” If you aren’t 100% sure, think twice.