Bonsai Bark did a nice series in the past couple of months on air-layering. If you have mature yard trees, a well-developed branch you need to remove, or would like the chance to start over with your nebari, then air-layering is definitely something to look into. Here’s the links:
Here’s an explanation of the process and why it works over at EverGreen Garden Works: What is Air Layering?
Some other resources:
If you have any other resources about air layering and it’s relationship with bonsai, send me the links and I’ll add them to this post.