The most effective referral technique is checking in with clients to see if they recognized the value of the work you've done for them. Here are two sample conversations:
Before the meeting: "George, I'm looking forward to our meeting in two weeks. I want to give you a heads up on one of my agenda items. I want to check in with you to see how our relationship is going–to see if there are any areas of improvement we can make and to make sure you're getting the value you expect from our relationship. If you like, think a little bit about this before we meet."
During the meeting: "George, you'll see the next item on our agenda is 'value discussion.' Let's put the market aside for a minute and talk about our relationship and the quality of our communication. Is there anything not working for you in our relationship?"
