The #1 source of clients, in any business, are referrals from past clients. Don't look at your client list as a source of a sale. Look at them as a source of many, many sales. Invite them to lunch. Tell them you're never too busy to give their friends and family the same great service you gave to them. They might not know someone right that second but mark my words--as soon as they hear someone mention real estate, they're going to think of you and send over a referral. You know the best part about it? It's no longer a cold-call at that point when you call the new client. You're able to start your call with, "I was having lunch with Mark and Linda Smith and they told me you were looking to purchase a house and possibly list your current home." That's a powerful intro to a conversation.
Second to that, contact your entire sphere of influence. Maybe you haven't sold some of them houses yet because they're just an 18 year old friend of a friend, the guy you know from the golf shop, etc. Well, guess what? They have parents. They come into contact all the time with others that may be buying and selling. The main point here is that you have to constantly remind people that you're an agent and that you want their business.