Ed's (Freakie frog) suggestion to use a snake bag is exactly what I do. A number of our snakes are too wiggly or explorative to stay still long enough for a good weight reading so I keep a selection of different snake bags or pillowcases handy for weighing in day. Like Ed said, just make sure you tare your scale with the empty bag on it first so you get a true reading of the snake's weight not including the bag.