I have here at the end of this post a great little tutorial on basic animation in After Effects starting with a brief description on parenting and the Puppet Pin tool. The second video is where he really starts to do the null parenting, so if you're good with the basics skip on ahead.
But for those of you who don't need a full tutorial here's a brief set of instructions:
In a nutshell:
1. Pin your subject and Name your pin(s).
2. Create a null object (Layer/New/Null Object) and Rename the null to match the pin.
3. Position the node to the same values as the puppet pin's position.
4. Shrink the null's scale.
5. Hold Alt (Option on Mac) and click the stopwatch for the puppet pin position. (This will open new options.)
6. Drag the new Pickwick (the swirly icon) over the Null that you're parenting to. Release.
7. In the new box (over the timeline) add this command to the end:
It should now say something like:
This makes the pins not go crazy.
8. Rinse and repeat.
Ta da! Now the null controls the puppet pin. You can now also parent layers to the nulls so that when you move the null the pin and the parented layer will respond. (Example: Head layer parented to a neck pin from another layer.) You can also create a chain of parented pins. (IE: Shoulder rotates causing elbow, wrist and hand to follow.)
And now the videos:
OTHER Puppet Pinning RESOURCES:
A Very Useful Page by Adobe
Puppet Pinning 101 Video Tutorial covers the types of pins and how to use them.