Just a quick post here, based on finishing a short run with 22 channels of wireless microphones.
My favorite position sonically is still forehead -- the "Angler Fish" position as one of my young cast called it. I know some people prefer to stick the mic at one side of the forehead or the other but I just go dead center (or at an available part in the hair). Resist the temptation to stick it as low as possible. Unless the actor has very long bangs or a wig, a tape-width from the hairline is plenty. Much of the sound it picks up is off the resonant cavity of the forehead. It sounds GREAT on good female singers, can reduce nasality on other actors, and doesn't usually need the corrective EQ of the other classic positions.
So. A narrow strip of tape on the wire, right behind the mic head. Cord goes up along the center of the head and is held with two hair clips or bobby pins. Depending on the hair length in back, you might add a wider strip of tape across the nape of the neck. Then down into the neckline of the costume.
There is what is known as a "halo rig" which is a bit of elastic thread sized to the actor that secures the head of the mic. Properly done, it is nearly invisible. I've never used one myself, though, so have no advice in making one.
The fall-back position is ear. Many actors prefer it for some reason (although my clever young cast also had a name for it; the "Parasite" mic. Because they felt it made you look like you had a bug crawling on you.)
In any case, resist the temptation to angle the mic down towards the mouth. Again, you are trusting the acoustic coupling of the body cavities. Also, there is a nasty resonance in the hollow of the cheek that makes the mic sound muddy and muffled and requires a ton of EQ to ameliorate. (Try humming and press your finger to your cheek. You'll feel that low-frequency resonance in your fingertip).
Instead, feel for the "cheek bone" (zygomatic) and place the mic there, a narrow tape-width from the sideburns (or whatever hair the actor has in front of their ear). The mic cable runs parallel with the line of the cheek, aiming basically towards the tip of the actor's nose, but is back, as close to the hairline as you can tape it.
Run the cable behind the ear and tape behind the ear: low, around the earlobe. Then pull the cable the the center of the neck, ask the actor to turn their head the other way, and tape at some arbitrary point about half-way between hairline and collar. Much depends here on how the actors wears their hair, and what the costume looks like.
In all cases, cables run OVER the furthest UNDER garment; under as much as possible so the cable doesn't get snagged, but NOT against skin (because skin conducts, and will drain RF energy.)
In some cases the voice is so weak you will have to run an ear mic low, bringing it close to the outside corner of the mouth, and tape on the cheek. Don't do this unless you have to.
There are a couple of series of microphones designed for this position, with an integral wire boom. The Countryman E6 (which I was using this show) has a couple of issues to note. First is that the wire boom is sensitive to handling noise. It is best to tape the mic to the actor's face close to their ear, but leave the rest of the boom with a slight air gap, so it is suspended in front of the actor instead of being in contact with their fact. Also, the connector at the rear of the ear clip makes securing the mic there problematic.
Countryman apparently will custom-make a set without the connector. Baring that, avoid putting tape across the connector itself, as it tends to rotate and introduce crackling sounds. The best option is taping the wire below the connector.
Do NOT let the boom drift until the mic is under the actor's nose! You will hear nothing but Darth Vader noises the entire show. Also do not allow it to go in front of the actor's mouth; bend the boom in whatever peculiar ways you have to to prevent this. Countryman does make a shorter boom for younger actors, fortunately.
The "Madonna mic" (personally, I'd prefer to reserve this term for a boom mic with a large foam pop filter on it) is a very different sound. It is more than anything like a telephone voice; very thin, lots of lip and breath noise. Reach for this position for two main reasons; because you want a specific sort of sound, and because your actor is so quiet you can't get enough gain before feedback without it.