The Missing Pets Partnership has assembled pages on what to do to find a missing dog or cat.
(Or ferret. I suppose that there are indeed people who would want to locate their missing pet weasel.)
It's pretty common-sense stuff. The one thing I learned, though, when it comes to looking for a missing dog (particularly in rural areas) is simply to look first upwind. Is there a lovely scent of carrion on the air?
(Hat tip to my sister, Sarah, who is always sending me dog links.)