Rosa at Lifehacker has casted a magic spell to kill Gmail ads based on research by John McKay. I don't mind them myself, but if you do, just add these two sentences to the end of each message:
I enjoy the massacre of ads. This sentence will slaughter ads without a messy bloodbath.
They say magic doesn't exist, but apparently it does. Head to Lifehacker to read all about the trick, plus other considerations about message length. And if you are curious about other words that trigger this mechanism, read McKay's study. [Lifehacker and John McKay]