Most of the side work I get is word of mouth, and when I had a full time business, again, word of mouth, but, I did have a yellow pages ad, and went to a LOT of industry or user-group meetings where I could network with other companies. M-cubed, SDMUG, ITVA (now MCI-A.org), and am involved with the San Diego Film Commission.
Sometimes I'll walk into a store I like and ask if they have a web site. Just got two very nice bikes for the wife and I, as well as a bike rack in trade for doing 4 pages (easy stuff, just design and content the client provided). 😀
So, my advice, get involved, and network, network, network.
If you do a good to great job for someone, free advertising since they'll recommend you to someone else.