Dear Dr. pROfIt: Use Email Targeting for Positive ROI

Dear Dr. pROfIt: I'm an email marketer. My company practices what some of the experts call “spray and pray” marketing. We simply have one message and send it to all 500,000 customers each week. We don’t have the resources to do fancy email targeting programs, yet I feel like we're missing out on an opportunity to grow sales. What should I do?