Make sure that you know your audience
Who is in your database? Take a look through the list occasionally to get a feel for the people that have subscribed to your newsletter.It helps if you personally know each and every person, you can get a better feel for what content might make sense to them.
Make it personal! If you have tow or three types of customer then create three different newsletters/offers. This can be as simple as slightly changing the content of one newsletter to suit each target market.
Make it worth their while. One way of getting a few extra sales is to offer products at a better rate/price for people who are in your email database. If nothing else you will get these people reading your emails more often.
We are all probably guilty of not taking the time to do the above at some stage. When it comes time to write your newsletter or create an email with your monthly specials we scurry to get content together and we forget our audience.
Taking the time to plan each newsletter is much more effective than just sending it.