PhoenixV, I often add my own poems to cards. Otherwise, I add something by a poet I've read recently or Elizabeth Barrett Browning or Robert Browning or Shakepeare. A poem of one's own making is best I think!
Elizabeth Barrett Browning
Sonnets from the Portuguese
XLIII. "How do I love thee? Let me count the ways..."
How do I love thee? Let me count the ways.
I love thee to the depth and breadth and height
My soul can reach, when feeling out of sight
For the ends of Being and ideal Grace.