The thermometer banner was a great idea, it worked really well
This wikipedia banner got the most click-through rates (2.85%). I can see why - it's very strong, and the body page is usually graphically very 'weak' Wikipedia Posts Report on Effects of Banner Appearance on Donors
the last fundraiser was fun, exciting even
I think once every two years would be a good average - or more importantly whenever needed.
edit/ I got that wikipedia page by clicking on random on the left there...