I used this technique in my Hearthstone Plaque and Chalkboard Conversation Heart Instructables but the hanger assembly seemed really useful so I thought it deserved its own Instructable.
Step 1: You Need
Wire for Hanging (non-rusting like copper or aluminum; sized to hold the weight of your finished product)
Plaster - mixed as directed
*This MUST be traditional modeling clay or Plasticine, not Play Doh or similar play clays. Play Doh is water-based. It tends to absorb moisture out of the plaster and turn into a big, gooey mess that can't be pulled out of the cavity easily.
Step 2: Sizing
Determine how wide you want the cavity to be. It should be fairly wide, allowing you to center the object when it hangs. This is especially important with unsymmetrical objects because you won't know where the center of gravity might be. Add an extra inch or more (depending on the size of your object) to this width and cut your wire to length.
The cavity should also be sufficiently tall, so the object hangs off the wire, not the top of the cavity.
Step 3: Creating the Hanger
Step 4: Installing He Hanger
Once the object has set, use toothpicks or other tools to dig out the clay. Finish the piece as you'd like and hang from your wire.