Listen to sad songs. I have a list of them:

Luther Vandross - Dance with My Father
Gary Jules - Mad World
Tiffany - Could've Been
Janis Ian - At Seventeen
Bright Eyes - Poison Oak
Bobby Goldsboro - Honey
Nancy Sinatra - Bang Bang
Supertramp - The Logical Song
Richard Harris - MacArthur Park
Plain White Tees - Hey There Delilah
Priscilla Ann - Fine on the Outside
Harry Chapin - Cat's in the Cradle
Loreena McKennitt - Greensleeves
Rihanna - Unfaithful
Jewel - Foolish Games
Liz Kay - True Faith (orig. New Order)