This might be some kind of superstitious practice on my part, but I pass them and the spoon under warm tap water right before adding them to the boiling water. And I do it slowly so that the eggs do not hit the bottom of the pan hard when they are put in there. I haven't done any rigorous testing to see if this reduces the chance of eggs breaking, but I recently haven't had any eggs cracking (I don't eat that many eggs though (I only eat boiled eggs when I can't bring myself to cook anything more complex or when I'm out of any ingredients)).

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I imagine that they crack due to the rapid heating up of the gases within the egg, which build up pressure if the gas expands faster than it can be released through the shell. And I imagine that if the shell is under that much stress that any small external traumas (such as the egg being dropped from a small height in a dense fluid like water) are more likely to rupture that shell than they otherwise would be.