It's easy. Just listen to what Troy White has written in his recent Word Wealth blog:
"It costs one-sixth as much to sell something to one of your existing clients than it costs to go out and find a new customer. And yet, almost every single company I talk to spends two to three times more effort on finding new clients than investing in old."
It doesn't take being a rocket scientist to figure out that you should be paying more attention to your current customers -- and selling to them -- than trying to uncover, attract, dig up, or otherwise snare new customers.
You should read more of what Troy White and the rest of his companion bloggers have to say at Duct Tape Marketing.