For many types of molds, you'll want your plaster bandage shell to stick to your alginate mold. Here are some techniques to make that happen.
Plaster Bandages Don't Normally Stick to Alginate
Nothing much sticks to alginate once it has set. The way to get something to stick to it is to embed something into the surface of the alginate while it is still liquid. Then when the alginate sets it will have something on the surface that will be able to grab the plaster bandage shell as you build it up.
More Plaster Bandage
One way to do this is to have some small pre-cut pieces of plaster bandage easily available. As the alginate is beginning to set, it will set from the inside to the outside. Take this opportunity to tap a bunch of small dry plaster bandage pieces INTO the outside surface of the alginate. They will be mechanically retained on the alginate surface so that when the first layer of the plaster bandage shell is applied the two plaster bandages will stick together.
Another thing that can be embedded into the surface of the alginate is a fibrous product used for stuffing quilts. It comes in sheets. Pull the sheet apart so it is just half as thick as it was originally. As the alginate is beginning to set, a sheet of this stuff is gently pressed against the alginate surface. When the alginate sets, the sheet is removed but it leaves behind thousands of little hairs. The plaster bandages can attach themselves to these fibers. Voila.