With traditional media I tend to sit back with the sketch pad in my lap, or I'll lay it flat on the desk. Sometimes I'll sit at the couch and use my laptop lap desk which has a bit of an incline. Then of course there's the awkward standing position when I go for a walk at a park or the zoo.
When using the tablet, I tend to run out of desk space (too much music equipment) and hold the pad in my hand or in my lap, because apparently I don't think playing the guitar is giving me carpal tunnel fast enough. Maybe a change is in order...