Try sending them content that's informative, entertaining, and relevant to them. Could be something simple as emailing them an article that they may find interesting.but what do you email them. I don't like to send anything plan specific because if I do they may just go to the plan website and sign themselves up.
Eventually, you're going to want to send them original content. Either write it yourself or pay someone to write it for you.
As for ideas, think about what are things that you are constantly having to explain and educated your clients about. What are the common misconceptions you hear everyday. Start there.