I don't think anybody keeps their cool all the time: the one thing SAHMs have working against us is the sheer number of uninterrupted hours we have with the kids. There's no other job in the world that requires you to be "ON" as long and as often as parenting.

What works for me: I say "Kindergarten teacher. Kindergarten teacher" over and over, and I pretend I'm that kindergarten teacher, full of cheer and happiness and secret knowledge of the craft cupboard. The kids tend to do a 180 when ever I go into kindergarten teacher mode, and before too long pretending to be cheerful turns into really being cheerful.

(Either that, or they're petrified that it's a three faces of eve deal and they want to keep me in this face before Sybil the destroyer emerges...either way, I'm cool with it)