The #1 Rule To Any Successful Newsletter

A newsletter is a tool that every business can and should take advantage of, but it is something that needs to be done right. By correctly utilizing a newsletter you can stay in contact with customers or clients and build a relationship with them. The relationship formed with your clients leads to return business , referral business, and it can help you create a following. 

In the world of real estate, referral business is a big part of getting more deals and new clients. By using your newsletter you form a relationship and gain the trust of your clients, even after you closed the deal with them. This trust and relationship often leads to a lot of referral business for you down the road.  But, this only happens if you do it correctly.
What To Avoid
Before we get to the #1 rule of a successful newsletter we need to go over what NOT to do. A lot of business owners use their newsletter to sell, sell, sell. This is the biggest mistake you can make. No one wants to be sold to over and over again, especially via email where spam is already a big problem. If you use your newsletter to constantly sell you get a big unsubscribe rate. 
Selling can be looked at several ways. Being in the real estate industry you are not trying to sell a product (usually), but constantly sending out your listings or homes for rent is a form of selling and it will lead to a high unsubscribe rate. This isn't to say you shouldn't include those in your newsletters, because you should, but you need to add more value than that. This is especially true if most of your newsletter subscribers are clients who already closed on a property and are not in the market. They will need more value if want them to stay on your email list. And this leads us into the #1 rule....
Always Add Value
When you provide your newsletter with value, they in return, will remain on your list, give you their trust and see you as an authority in your niche. As an agent, property manager or other real estate professional this couldn't be more valuable. Anytime that person needs a real estate professional you will already be in their mind. This holds true with their friends and family as well; when their friends and family need a real estate professional they are likely to refer you.
What To Include
There is a lot of information you can include to add value. Things like local news, home improvement how-to guides, "what to except" articles for first time buyers and sellers, other how-to guides on subjects like getting your full security deposit back, and anything else that will interest anyone. The key is to include things that would spark interest in anyone, not just people currently buying or selling homes. Since everyone lives in a home, whether they own or rent, those subjects are things everyone can relate to. This way you relate everyone, regardless of whether or not they are currently buying or selling. 
I know this works because I am currently on an email list of a real estate agent who helped me find a home. After we moved in to the home I remained on his email list because he would include interesting news about the local community and include interesting articles about maintaining a home. Next time I am in need of an agent he will be the first person I will call because I have looked forward to his emails and gained value from it. I know he generally cares about informing his cliental about the local market and isn't just focused on selling, selling, selling. He does include some listings and links to video tours at the bottom of his email, but that is not the focus of his email. As long as you add value to your cliental, you can easily grow your real estate business over time.