Word of mouth.
HEY FRIEND COME OVER AND PLAY THIS NEW MARIO KART DEMO
FRIEND IS HAPPY AND SAYS OH MY GOSH GOLLY GOSHKINS I WANT A WII U NOW TO BUY THAT GAME WHEN IT COMES OUT
And all demo's can be buggy, this is just a cop-out.
Let's be real here, how many gamers actually have friends who aren't gamers?