Say nice things about your donors. Really, it's a smart thing to do. A study reported at the Neuromarketing blog Flattery Will Get You Somewhere, shows the surprising power that even baldly insincere flattery:
... even when we realize we are being flattered, and "correct" for that when we think about the flatterer, there is still an underlying positive impression that can be strong and long-lasting. This subconscious positive impression ... was found to influence behavior even when the subjects consciously realized that the flattery was insincere.
Let that sink in: When people are flattered -- even when they know they're being flattered, even when they know the flattery is fake -- they respond more positively.
One of the many nice things about donors is you don't have to be insincere to flatter them. They are self-demonstrating excellent people. Just pointing out the truth about them is enough.
So go for it. Flatter away. Your donors more than deserve it.