Russell Napier wrote the book on identifying bear market bottoms (really) and Alphaville provides selections from an interview he recently gave to the FT. Fascinating stuff, if you like reading your news through Roubini-tinted glasses.
The concept of identifying any sort of local minimum without the benefit of hindsight is inherently questionable, but the use of indicators (such as P/E or q ratios) to identify what is most likely not a bottom may hold more water.
But to sum it all up, I can't do any better than www.isthisthebottom.com.