A combination of online and physical advertising.
Websites that accept open house info is a good start, Craigslist has done right by me several times bringing in traffic that might have missed other advertisements, if the house is waterfront I'll post it on boating and fishing forums. Most of the time I'll just compile a few photos into something like Publisher or powerpoint, type in the relevant data, and save is as an image that I post on those sites.
I'll also create flyers and post them in local supermarket ad boards and college rec/student centers, balloons and lots of signs the day of the open house. If I'm not getting much traffic, I'll provide a lunch tray. Every extra bit helps, especially in this market.