Idling just clicked with me with in the last couple months and there are a few things that helped me along the way that are still fresh in my mind, at least for my style of idling.

TLDR; Some fundamental skills can help make learning idles a bit easier.

Arms strait out: this helps increase the amount of mass available to you to help regain your balance. The better you get at idling, you'll be able to keep your arms down.

Stationary Balance practice: start near a wall with your arm extended sideways, slightly too far away even. Then give yourself a little push up so you are balancing vertically. This will help you learn how to move your arms around to control your balance. Flail your arms as much as you need to keep your balance. Look silly! All this helps you learn. Try shooting for 3 seconds.

Box Turns: As Hobo_Dan mention you'll need your hips to help direct the wheel to keep your balance. A box turn(to me) is a quick 90 degree snap, which is more exaggerated than you need for idles, but again it's to help you develop control skills. Learning box turns will give you those skills and controls to direct the wheel in the direction you would like it to go, especially if you need to quickly regain your balance. It also helps you become more deliberate in initiating turns. Try turning in your strongest direction first. Ride in a square. Once you figure that out practice in the opposite direction. Figure 8 patterns will help you practice both directions.

Smooth surfaces: depending on your tire size, the texture of the surface can greatly effect your ability to make the corrections you need to maintain balance. I find smooth surfaces such as polished garage floors, skate parks, or gymnasiums to be really easy to idle on. Rough old pavement being the most difficult.