I put another tagline on the back of my card. At the bottom my website and email address is listed. I left lots of white space for notes to people who receive my card. Lots of networkers in my area like to be able to put some notes on the card to remember you. Although, you should ask permission BEFORE writing on the back of someone else's card. I think you idea of a quote that has inspired you is a good idea but I would stay away from song lyrics. This is just my opinion. You could also list some of your services, IT it doesn't take up too much room. In addition, the back of the card should have the same orientation as the front - both either landscape or portrait.
Hope this helps!! Good luck!
Janet
eBusinessOnlineSupport.