Watch on youtube.com
I've been picking up the Stage 3 88 (which is, to be honest, pretty heavy) by holding the wood "cheeks" on both sides. Of course I've never clamped down on the keys themselves to pick it up... that much I could already tell.
And then I watched someone who actually knows how to lift it: slip your hands underneath, hold the back and "forklift" it.
But remember, always lift with your legs and not with your back.