I learned to idle rather more recently than John, but totally agree with him - I found it even more useful learning the motion by holding on to something when learning with my non dominant foot as it felt a lot less natural. Though I also disagree with the suggestion that you’d need to learn to hop the other way round - I could hop either way round way before I could idle at all, but the two skills don’t really have any connection (TBH I’m probably better at idling wrong way round than hopping wrong way round now as it’s something I tend to do a lot more of - as suggested it gives the leg doing all the work a rest).
Don’t you just love when a relevant thread pops up 
I’ve been learning to idle during the winter months that the weather’s been too crap to go out and ride properly - I started learning on my 29er with 137mm cranks, but went to my ol’ faithful 24" to make learning easier. I’m finally starting to get the hang of it (I can pretty much keep it up for almost as long as I would want to, but I tend to rotate on the spot a lot).
I’m curious to get tips on learning to do it on a bigger wheel, and possibly with short cranks - would it be a lot different? I’m not talking, 36" with 100mm cranks-level, but I’d love to learn to do it on my road 29 eventually (114/137mm cranklengths).
Figure how to make your rotation go the other way. Once you figure that out, you can decide to rotate or not.
Slow motion. The bigger the wheel, the slower the idle, at least to do it comfortably. To learn, I’d start by learning to slow down your idle on the 24", and when that gets boring, try doing the same thing on the 29". Start by holding onto a support, and then start letting go.
I can idle more easily on my 45" wheel than a Coker, but this is for two reasons. The 45" has a hard, solid rubber wheelchair tire, so it twists easily (more than you generally want to), and it has 165mm cranks. My 36" has 150mm cranks, but the tire is much more grippy, which makes it harder to make those twisting corrections. Basically I never idle on my 36". If I’m stopped at a red light, I’m off, getting some crotch circulation. People in cars like to watch you freemount anyway (at least I assume that). 