Here’s a site for some reference on facial expressions. It’s pretty helpful just to get an idea of where wrinkles go on a character and to get to know the areas of the face along with the proper name.
http://www.artnatomia.net/uk/artnatomyProgram.html
Here are some other books that I definitely recommend getting for really good facial expression.
This one is just filled with hundreds of different expression’s of people from different age groups and races.
“Facial Expressions: A Visual Reference for Artists” by Mark Simon
While this book going into the anatomy of the face, basically a deeper understanding of why and how the face the way it does.
“The Artist’s Complete Guide to Facial Expression” by Gary Faigin
http://www.amazon.com/gp/product/0823004325/ref=s9_simz_gw_s0_p14_i2?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=16115YFKWSD8HG721W37&pf_rd_t=101&pf_rd_p=470938631&pf_rd_i=507846
Though the books I have linked are great for facial reference, the best reference you can get is simply by looking in the mirror and analyzing what moves, stretches, wrinkles, and relaxes.