Your existing customers should be your first priority. I know it sounds obvious but you will hear it said repeatedly and for good reason. I have heard it takes 10 times more resources to win a new customer than it takes to serve an existing one.
Serve your customers right and they will come back for more and more. They may even tell other potential customers how wonderful you are. This is much more effective and a lot cheaper than most marketing communications.
Serving your customers is not just for the service industry, it is for everyone. Take an example of what happened to me recently.
My bank rang me up to ask if I would make an appointment to discuss a small (but significant to me) bond that was about to mature. Definitely a good bit of customer service. I said yes, as I did not really know what to do with the money, interest rates being so low.
On the day of the appointment, “Alan” phoned to say the branch computers were down. He could not see me as agreed but he would ring later that day to arrange another appointment. Of course, he didn’t. What he did do was prompt me to go on the internet and look for other deals. I withdrew my money and gave it to another bank. Not much loss I know, but if they want my custom back, they are going to have to pay dearly for it!