This technique is beautifully simple and extremely quick.  Although not perfectly accurate, it can be an excellent guide for getting your general bearings.

The ‘crescent method’ works as follows:

Mentally draw a line that connects the horns of a crescent moon and then extend this line down to the horizon. In northern latitudes this will give an approximate indication of south.  It works best when the moon is high in the sky and not too near the horizon, when significant errors are possible.

Sources:

• http://www.naturalnavigator.com/find-your-way-using/moon
• http://blogs.msdn.com/b/mattlind/archive/2007/12/20/how-to-find-south-using-the-moon-shadow.aspx