How to add battery backup power source to a circuit?
The key to these circuits is that you have to find a way for your circuit to turn itself on provided there is no voltage applied from the external power source. Many very basic reverse battery protection circuits (that go beyond a series diode) can actually be used with litte to no modification for this purpose - I would suggest starting there. Take a look at figures 2 and 3 from his application note from TI:

http://www.ti.com/lit/an/slva139/slva139.pdf