Or, "Warts and Angler-Fish."
A lot of people have been asking, so I dedicate this post to it. Would do better with pictures, I know.
Cheek Mic (what some of my younger cast call the "you've got a parasitic infection" look.) If there is nothing unusual, like glasses, bushy sideburns, a hat, a mask to get around, this is where and how it goes;
Feel for the cheekbone -- the zygomatic. It starts at the hairline at roughly the lower margin of the eyes, and for the first centimeter or two makes a line that points towards the philtrum (the space between upper lip and nose). Jaw muscles originate just below this bony prominence; press a finger against your own cheek and make a chewing motion and you will feel how on the cheek, you have movement, but on top of the cheekbone, the flesh remains almost still.
Starting with the microphone under the shirt or blouse and coming up through the neck hole in back, pull the mic over the top of the ear and stretch it along the zygomatic -- just on top or slightly below in the notch. It should be along the same line as the bone, making a fairly straight line as it points to the margin of the upper lip. Avoid the temptation to angle it lower.
Pull the mic out until there is barely one width of tape between the head of the mic and the start of the hairline (aka the sideburns). Tape there. For younger cast I buy 1/2" tape or tear the 1" in half. For women and children, you can usually brush aside much of the stray hair in front of the ear to make sure you are not putting tape on top of hair.
So that's four things to watch out for; don't pull the mic out too far, don't tip the mic down or otherwise allow it to get on the soft part of the cheek, don't get tape on the head of the mic, and don't tape on top of hair (it is uncomfortable for the actor and doesn't stay on, anyhow).
On most actors, dress the mic behind the ear and tape once behind the ear; when the space is large and clear, actually behind the ear a bit above the lobe -- I've found a narrow strip of tape done at an angle works well -- and when the space is small or there is a lot of hair, just below the ear on the broad mass of the sternomastoid itself.
For actresses with lush hair (particularly girls) you can save them tape behind the ear and use a bobby pin or hair clip right where the hair tucks over the back of the ear.
The last piece of tape in the typical three-piece arrangement is on the back of the neck. I used to recommend low, around the 7th cervical vertebrae, but I've changed that now to a 3/4 position, along the mass of the trapezius and just above the "V" where shoulder line meets neck line.
Okay, I've given a bunch of exceptions here already, but really, for twenty actors you can go through 18 of them with the basic three pieces of tape, slap slap slap. I've done a cast of twenty myself in under fifteen minutes.
Hair Mic. What one of my younger cast called the "angler fish" look. Also when done wrong can look like a caste mark. Seriously, there's not enough sonic difference between just down of the hairline, and inside the hairline, to make it worth staring at a microphone all night.
The mic goes on the forehead. If the actor has hair with an off-center part, this may give you a better place to lead it, otherwise just go center. Tape just behind the head of the mic, and as close to hairline as you can get...if you have to tape. For most actors, it is better to pull the mic up until it is just barely peeking out, and secure it with bobby pins or hair clips.
Work the mic up along the top of the head and back, pinning as you go. The slowest to dress are actors in natural hair. With wigs, you either have a wig cap, or the actor's own hair in coils, and it is easy to pin to or weave the mic inside.
Particularly, girls with wigs or "trousers" roles will have the bulk of their hair pinned up in a bun or french roll. You can pull the mic through that and let it dangle in back. Then all you need to do is pin the length up to where it meets the hairline in front.
When the hair is not supportive of the fragile neck area, this will be a piece of tape.
Hair mics take longer, and take more experience and judgment in figuring out how best to deal with each individual actor. The trade-off is that they, of course, sound better.
Lapel Mic. Completely inappropriate for most live theater, but you may have to do it for a presenter or work a lecture or talk some time.
No tape. The mic goes into a clip, which clips to clothing. The trick is to get out of chin shadow; don't go on to a high collar. As a rule of thumb, feel for the top of the dagger-bone -- below the clavicular notch. Or the other rule of thumb...imagine the microphone is a little light, and it should touch the lips without the chin casting a shadow on them. In most cases it looks nicer on clothing to be to one side or the other, on the inside edge of the lapel on a sports coat or similar.
In the case of, say, a turtleneck sweater, make a judgment call about whether you'd prefer to be watching a puckered sweater with a mic attached in the middle of the fabric (the thicker, looser weave, and more colorful the sweater, the better this works), or listen to a poor voice from a position that is up too high.