Roy Williams shares 10 Cheap Advertising Ideas in his Monday Morning Memo.  My favorite:

10. Spray-Painted Signs. In the early 1970s, "Hamp Baker says Drive with Care" was spray-painted on car hoods salvaged from crumpled automobiles, then those hoods were tied with bailing wire to barbed-wire fences across the state. Nobody in Oklahoma had ever heard of Hamp Baker, but his name was soon a household word. When he ran for public office, he won by a landslide.

If I had a personal injury practice -- especially in a rural area -- I'd think seriously about giving this one a try.  Just make sure you have your state's disclaimer painted somewhere on the hood too.  ;-)