Keeping it fresh is a challenge, but a fun one. Do some research (together or separately) by reading stories and watching some of the better porn out there (almost anything from Kink.com is good) and making note of things that interest either or both of you. Make her describe some of her own fantasies and take notes on how you can actualize them. I usually have a few ideas of where a scene will go and what I want to play with, but then let it flow as we move through it. Create a training manual that includes goals spread across a number of sessions, like increasing stamina, or pain tolerances. Withhold her spanking and use it as a reward for pleasing you in a method of your choice. Focus on some of the mental aspects of BDSM.

As for spanking, try different implements and tools. And don't forget about your needs and desires as well. After all you are the Dom!