A combination approach of focusing on a marketing niche, providing a limited but free useful resource to demo your skills and newsletters about that resource works for me. Decent Google visibility doesn't hurt either. I just started posting in forums like this too.

Establishing a marketing niche over time is critical. Offering general web services can make you blend in with everyone else offering design, marketing, programming, etc.

The clients that pay better for me typically are small 1-4 person businesses in a similar marketing niche, using the same software and who have an established growing client base already.

These types of client I approach directly using email and voice. Essentially your competitor can make a great client if you bring something to the table that helps you both sell more.