If you have happy clients, ask them proactively every time you can talk to them. Fact is, if they are happy with your results, they would be more than happy helping a friend and selling the idea of your SEO services for you.
That's how I'm growing my business right now, through referrals. It's a very simple sale if you compare that to cold calling, direct mail and other form of marketing.
So, ask for referrals every single time, happy clients more than happy to recommend you would sell for you.
Don't underestimate my advice, go and do it and thank yourself later.