What I am doing when I make a lifecast mold looks quite simple, but isn't easy in practice. I am very experienced working with plaster in my regular business, architectural casting.
After the posing and the greasing (both of which have tricks too), comes the first coat of casting plaster, which defines the shape and size of the mold and thus the finished piece.
The first layer is smeared all over the area I want to cast. At first it has a beautiful lacy look, caused by water not mixing with oil (wet plaster over vaseline, that is.) As it thickens slightly, the plaster will form a thin, unbroken coat about 1/16 " thick.
Many small pieces of 1-oz. fiberglass mat are prepared beforehand. These are dipped and squeezed in a bowl of regular casting plaster a few at a time, redipped and laid over the thin layer of wet plaster on the model. They may slip right off-- I have more tricks. These should overlap a bit, for strength. They need to be lightly smoothed down. A helper is necessary on ambitious pieces, and appreciated always. They mix the bowls of plaster and dip the mat, and after they get the idea of it, help place the mat on the model.

The diaphragm is the large area on everyone's torso below the ribs and above the pelvis, a diamond shaped area of muscle. The model's diaphragm moves in and out, stretching the thin wet plaster on inhaling and vice versa. The plaster mat layup moves with the diaphragm until the plaster takes a set, and at that moment, the whole body had better be laminated, because the tiny movements of any living model will crack the thin layer like a cracker.
So the belly will always be captured at its expanded point. This isn't unflattering, anyway. Models need to learn that they cannot keep it sucked in for very long. My technique takes about 25 minutes and the model has to get a sense of how muscles behave in that time. That's why so many of my molds are faulty on the first attempt. (That 25 minutes does not count the time spent placing plaster gauze over hair.)
Cool plaster takes longer to harden than warm, but warm feels better on the skin. That's why I prefer to lifecast in the summer-- cool gives me time, and feels pretty good in the  Memphis heat.
After that first crucial layer, further layers, differently tinted, can be added for strength. By using different colors, you'll know where you've already been. The edges especially need strengthening, and so do the areas on which the mold will rest when you are working on it later. They can be reinforced off the model, of course. Just be sure to lay the fresh mold on a soft foam pad.

My worst mistakes have been painful for the models, or just wasted our time. Not everyone can model; some people can't stand it, or just can't keep still at the crucial moment. Assistants, often models' boyfriends or husbands, have done things I didn't catch. We nearly tore a girl's ear off when the clay behind her ear (trick!) fell away, and we cast her without it; and I did capture my first model's pubic hair: I told her to grease up well down there, but I was too shy to check.

 I was able to break the plaster prong which formed where it slid down between her thighs. Now I grease what needs greasing, and use plaster gauze over all thick hair in the field of action. Body areas which are not to be included in the piece should be shrouded with plastic or cloth to keep them clean. 
See About Modelling, too.

Here's the big plaster secret: Plaster takes a set, and then it starts heating up. First, the set. It could be easily scraped around, but it isn't a liquid anymore. Then, the heat. People ask, How hot does it get? That's a trick question. Each cubic centimeter of plaster puts out just so much heat [calories]. So the thicker you work, the more heat you (the model, really) must deal with. 

So the big idea in my technique is this: That first layer has hardened, and is starting to heat-up. And there you are, covering it with new, cool plaster. The heat goes into that second layer, warming it and causing it to harden sooner, and cooling off that first layer by so doing. As plaster has its exothermic reaction, it grows vastly stronger. It's crystallizing, actually.

The thickness of the mold is still remarkably thin, maybe a quarter of an inch. Where it goes into a body crevice it will be thicker. I take the mold off the model as soon as the second or third layer hardens (maybe we missed some areas with the second layer). More tricks are involved. That warning about the plaster slashing your model makes sense when you look into a mold -- plaster which hardens in a body crevice is that sharp. The model is usually fascinated to see the mold, and glad the hair-yanking is over. She keeps most of her hair but a few do pull out. Her skin may be bright pink but will quickly return to normal. There will often be small bruises on the nose where eyeglass pads normally press in, for some reason.

If you need to make a complete full-round bodycast, you'll need to have made nice smooth strong edges. You'll go around to the back of the model, paint the plaster edges with oil soap solution, and start again. I don't care to do this very often.

I think that as the artist, my work is in choosing the pose and presenting the viewer with one bas-relief vision. But if it were easier to make a full-body casting, perhaps my vision would be different!  Fred Reenders has this down, with a completely different technique. He likes smooth, marble-like surfaces, while I prefer real skin texture. I guess you come to prefer what you're good at doing.

The mold invariably needs some work. Air bubbles, big and small, are in the mold surface. They need to be filled carefully. Often there are a few cracks and ruptures to mend and sand. The edge needs defining; this is crucial to the look of my pieces. Remember reinforcing the edge? If you missed some spots,  now you catch them.

When the mold has fully dried, it can be cast. To cast concrete, it needs to be coated with form oil. For plaster, use oil soap solution. For polyester fiberglass (FRP), shellac and wax it. To cast wax, for bronze, generally you cast in plaster, make a thin rubber mold of the finished piece, and pour wax in that mold. For clay, just work in the clean mold; see my ceramics page.