Referrals, referrals, referrals.
I've spent money on ads (not much though, and I might revisit it one day), I've redone on my portfolio website endless times, tweaked the SEO to the max before (which got me a small heap of traffic at the time), but always my next client comes from my previous client.
These days the web landscape (webscape?) has changed so much that I now only keep the portfolio website up as a formality; It gets almost no traffic except when I mail the link to someone to see. When work slows down, I start looking at which of past websites I've done need some upgrading, and start calling those past clients to sell them on improvements or new features. I ask everyone I've worked with to please tell their friends about me.
I wish I could give you a more helpful answer, but for me it's always worked way. It's a little... organic? It feels like I don't have a lot of control over the situation because it's not as mathematical as run-an-ad-for-two-weeks-and-get-two-clients. But one thing though, looking at my accounts over the past few years, my income as been pretty steady, even though my "marketing" hasn't been very hands on. That either means I've been doing something right, or I've just been really lucky. Maybe it's a little of both.