For all types of plaster hangings I use a piece of wire bent into the shape of the Greek letter Omega. As the plaster sets I position the leg portion into the plaster at an angle to create a hanging loop. Larger hangings should have 2 of these.
For smaller hangings some folks like to drill a hole in the back to fit over a nail, but I find this does not work as well and increases the chance of cracking the plaster.
Most of the time this is caused by failure to lubricate the outside of the mold before demolding. When ready to demold, dip the entire mold in water, then rub one drop of dishwashing liquid over the entire outside of the mold. The surface is now slippery enough to slide easily over itself. Failure to do this will normally result in a ruined mold.
If well cared for, the average life expectancy of latex molds is 40 to 60 castings. Molds with severe undercuts (i.e. that need to be stretched further) will generally not last as long as those without undercuts (such as a pyramid).
Can I allow my castings to remain in the mold overnight?
You could, but it is not very good for latex molds, other molds are less sensitive. Castings should be demolded as soon as hardened to minimize contact with the mold. Generally when th mold gets hot to the touch it is ready for de molding.