Place them on the board, left one to the left, right one to the right.
Seriously though, do what is comfortable. I haven't noticed too much difference in pop when I moved my stance further out, but the landings seemed softer. Some may say to just put them as far apart as possible; wide base is good for stability (like in most sports involving impact and balance!) but if you go too far it is no good. As far as switch, keep them symmetrical or close to symmetrical so you can easily go both ways.