I have a fairly extensive 'lessons learned' section in my
ride report for the 2012 Lewis and Clark Ultra.
You'll probably make plenty of your own, but learn from my mistakes.
Regarding training, I found the book
Distance Cycling by John Hughes and Dan Kehlenbach to have several helpful training plans.
Most importantly, have fun. I approached my first 24-hour as a challenge just to finish, didn't get
too wrapped up in competition.