While you may think you know your strengths your customers may be buying from you for completely different reasons. It may be that you have the best price, that you are local, that it is convenient or that your product fits best with what they do. What you have to do is ask a few of them and then highlight the features which reflect those reasons in your flyer. Send it out with invoices, with quotes and with proposals. Many of your customers will not be thinking about you except when they buy the one specific product they have always bought - your flyer raises your profile, makes them think of you in a different light and reminds them to buy from you.
2. You can send out a flyer to get a different kind of business from your existing customers. Although you already have the names and addresses this is more difficult because you don't know yet how these customers will react to the new products or services. What you need is some tests.
Since they are your existing customers, you must know some of them very well. Identify those and have some informal discussions with them regarding your plans. Ask them how they would react, what they would like about your new products or services and why they would buy them from you. Then you can proceed to develop your flyer accordingly.
3. Finally, you can send out a flyer to try to obtain new business from new customers. This is the most difficult because you don't know who these new customers will be or what they will like about your product but it is important to proceed in an organized, planned way to maximize your chances for success.
Again you'll have to talk with potential customers to find out how to target your flyer. Identify and group potential customers by reasons why they might be interested in your product and try to find personal contacts which belong to those groups so that you can bounce your ideas off someone who can give you guidance. It's useful if you start with groups which have some similarities to your existing customers and you can build on your experience there. While this type of business development is the most difficult, it can also result in huge growth if you're successful.