Since it is so obvious when a Monarch is going to eclose, I sometimes take them out of their enclosure and tape them to the bottom edge of a kitchen cabinet so I can see them up close. I tease the silk web off of the top of the enclosure and press it together.
Then I fold a piece of masking tape around it, and tape that to the cabinet.
I used to use a thread and tie it around the cremaster like others do, but once I pulled too tight and broke the cremaster off. :(