I have always found going on my stomach to be most comfortable, but then I am more prone to leak so I have to limit that, depending on the diaper worn. For capacity purposes, standing yields best results for me. I have just recently been experimenting with premium diapers, to go while on my back. In the past, due to variety I reasons, when on my back, I would often find the trickle going up and then around my sides, eventually causing a leak. I think either the better taping method or diaper itself has finally allowed me to experience wetting while on my back. It still more forced this way, but I get the better capacity. My favorite is still on my stomach.