For me, word of mouth, locally speaking. Less than 10% of my traffic is local, but 90% of my work is local, so getting to know someone face-to-face is huge. I think sponsoring local community-oriented charitable events, advertising in the local paper, etc. makes a big difference.
Word of mouth is the best refferal. But also having a site that ranks high on Google is good. I get a lot of public relations photography work because of it - mainly from companies outside Merseyside looking for a straight forward job and don't want to pay travel expenses to their usual photographer.
Obviously word of mouth, referal and advertisement plays a big part in obtaining clients but I have recently been gathering alot of clients through the means of Social Networking. People underestimate the power of social networking and the valuable contacts you can gain from it. For example, Twitter can gain you very useful contacts that later can become potential clients. Flickr as another example, put your work on flickr and gain contacts and from there build up your contacts and you will start to see potential clients. There is a thousand ways of social networking and if you find the right solution it can prove to pull in over half your clients than what you would get from word of mouth and advertisement.
Has anyone here ever ran a press advertisement that led to work?
I did it once, total waste of money - just one enquiry - that never converted.
Maybe it was my ad.
The best clients come from word of mouth and Google. I do receive a lot of enquires from site referrals. You know, putting 'website by Artworking' at the bottom of your websites but they are so difficult to convert in to clients.