Oxygen is the enemy of beer, except at the very beginning, to help the yeast get off to a rocking start. Whether the instructions say to "rock the baby" or simply to aerate, I've historically just guessed as to how to accomplish this without thinking much more about it. I'm not about to invest in an oxygen tank! So what do I do? I shake the carboy a few times, trying to get a good froth going. I pitch the yeast and then shake some more. Simple enough right?
What do the experts say? Here is Greg from Wyeast.
My take away is this. 15ppm = shake for 45 seconds. Easy enough. This is also perhaps another reason to leave some head space in the carboy...better splashing while aerating.