Just like any other animal i'd leave it be if it was having a shit, eating or just woke up, if you dont know the ferret that well. If its a new ferret, feed it up before you handle it, its less likely to nip you then. Apart from that just handle as much as possible, i can pick mine up still sleeping ( i have a narcoleptic ferret!) and wake them up in my hands and they are fine, hold them upside down, folded up, in a ball, under my arm, in my pockets, on the shoulder - whatever i like. Once they are used to you they are bomb proof little things. Mine particularly like standing on your head.