There's a lot of good advice in this thread. Let's see if we can come up with some basic rules that put it all together.
It's a true cliche, "We eat with our eyes." So making food visually appealing is part of what a good cook does. You already know this, it's why you started the thread. But it's a good idea to start with first principles anyway. Since a plate is a visual composition, it seems logical that the basic rules of visual composition apply. The plate sets a mood and a style, The plate identifies what's important.
But before we get into specifics Good plating starts with attractive food. Most attractive food starts with good knife work. It's important to be able to portion, trim and slice food neatly. If, as in most home kitchens, the sharpest knife in your kitchen is a serrated steak knife, and your "go to" knife is a dull paring knife -- you're starting at a distinct disadvantage. Not nagging, just sayin' is all.
Sometimes, it's best not to plate. Most family meals are better served "family style."' That is, the main course(s) and side dishes are placed in large bowls, and appropriately garnished. The serving bowls are brought to the table with appropriate serving pieces, they're passed, and each diner helps her or himself. This works very well with large portions, or when different diners want different sized portions. A filled plate, family style, is generous. A filled plate, "plated," is sloppy. A small portion for a small eater, family style, is appropriate and thoughtful. A small portion to a big eater, plated, is stingy and careless. Why deal with it? Let them serve themselves.
Large parties are generally best served buffet style. A good rule of thumb: If you have more than twice as many diners as courses -- buffet. The idea is to get the food out hot, and allow the diners the freedom of portioning their own food. Buffets tend to be less formal, so that's another consideration. So called "silver service," involves carrying serving dishes and serving pieces to each diner and either allowing them to serve themselves from the held dish, or serving them. Without waiters, silver service doesn't work too well.
Plating large quantities onto platters or into bowls is usually done as follows:
Small pieces, such as canapes, are arranged more or less regularly onto large platters allowing some space between them creating an impression of elegance; or are mounded creating an impression of abundance. When using a platter for this or any other purpose, it's important not to overcrowd. There must always be space along the edges.
Slices are either "shingled," or "fanned." An large, unsliced piece may be plattered with the slices. For instance, 2/3 of a pate de campagne (meatloaf, OK?) may be sliced, shingled, and leaned against the remainder -- as though the whole magically sliced itself. No matter whether shingled or fanned (shingled in an arc) the slices should run through the center of the platter.
Portions, as in portions of fish, are arranged more or less regularly and geometrically, each appropriately garnished. Arrangement focuses around the center of the platter. The center of the platter should be filled with the main.
Accompaniments, if served alongside a main on a platter, should be arranged along side. It's more attractive not to completely fill the sides of the platter, but to leave some space
When sauced with a thick sauce or gravy, slices or portions are somewhat under-sauced, with the sauce running as a ribbon down the middle of the slices. The remainder of the sauce is presented alongside in a boat. When sauced with a thin sauce, the portions or sauces are served on top of the sauce. Again, the remainder of the sauce is presented alongside in a boat.
Certain dishes determine their own plating on a platter. Take turkey: The drumsticks are placed at one end of an oval or rectangular platter, angled to a make a V, with the handle end extending out beyond the rim of the platter and the meaty portion facing in; the wings are placed at the other end, in a similar placement. The breast meat is sliced neatly and shingled down the center of the platter. The thigh meat is sliced as neatly as possible (usually not very) and mounded along side the breast meat.
Dishes in which liquid is an integral part, such as braises and stews, are usually served in bowls. Accompaniments such as veg and starch are also typically served in bowls. Bowls are served well filled. Garnish is kept to a minimum. For instance, a sprinkling of an appropriate minced herb, or a dusting of a colorful seasoning such as paprika.
Appropriate serving spoons, forks, ladles, tongs, etc., create ease and elegance.
On to the main course -- Plating individual plates
Learn from the masters and the chumps. Martha Stewart can plate. Cat Cora can plate. Emeril Lagassie can cook. He can cook the other two into the ground. Separately or together. But he can't plate to save his life.
I. First rule: Keep it Tidy. Don't Overcrowd the Plate.
A. Always leave a little room around the edge.
B. Negative space means something. An almost filled plate means "comfort." An under-filled plate means elegance. An overfilled plate means "messy." A plate with one small thing on it, means STAR POWER -- you better produce.
II. Second Rule: The main dominates the center, but does not need to be exactly centered.
A. The less you have on the plate, the closer to the center your subject should be.
III. Third Rule: Sauce visually. Pass extra sauce, if needed. No, that's not right. Let me rephrase. Pass extra sauce, or else.
A. Don't hide the meat in the sauce.
B. Don't drown the plate.
C. Lots of different things work, don't obsess.
D. Nap thick sauces -- a ribbon trail just slightly off center, oozing to a small puddle next to the main.
E. Pool thin sauces -- beneath the main; or pass them separately. Pooled sauces mean very small and appropriate accompaniments. For instance two or three baby carrots, and a two tablespoon sized portion of cauliflower puree.
IV. Fourth Rule: Styles change.
A. The current trendy style, whatever it is, sends a "fine dining" message. The most recent trend is vertical plating.
B. Older styles are fine too. Often your food will dictate a flat presentation. You can't put a thick pork chop or slice of roast beef on top of or leaned against mashed potatoes and still have it looked good.
C. Don't worry about TV dinner presentation. As long as the highlighted portion of the plate dominates the center, you'll be fine.
V. Fifth Rule: Keep things tidy. Don't overcrowd.
VI. Sixth Rule: Garnish minimally and appropriately. Keep things tidy.
A. Use fresh herbs. If the main or one of the sides is herbed, use only that herb, and only enough to "refresh" the herbs already there. Although parsley is (more or less neutral), too much is too much. Keep a light hand.
B. Colored salts such as pink Hawaiian or Gray Breton are very nice. Adjust the dish's seasoning appropriately.
C. Paprika and chili powders.
D. Only use diced pepper, etc., as garnish if you can cut a mini-brunois (a regular cube of about 1/16" square per side, or you can make decorative cuts. Otherwise it just looks sloppy.
E. Chive stalks are very nice.
VII. Seventh Rule. Accompaniments in Small Portions. Don't Overcrowd the Plate.
A. Doesn't need much explanation.
VIII. Eighth Rule. Keep things tidy. Don't Overcrowd the Plate.
IX and X. Same as Eight
A. Sensing a theme here?
Let's step outside these rules for a second and look at a few examples:
1. Imagine a stemmed champagne glass (coupe) filled with chocolate mousse garnished with chocolate shavings. Taken by itself it's a good looking dessert. Put the martini glass in the center of black dinner plate, and it's elegance incarnate. Push the glass off to the side of the plate, and it's ridiculous.
2. A slice of rib roast, mashed potatoes, creamed spinach, pieces of Yorkshire pudding cut from a large pudding, "au jus" gravy. How? The roast is plated with the curved top facing the outside of the plate -- curve echoing curve. The 1/3 line of the beef covers the center of the plate, so the beef is center stage. An appropriately sized portion of potatoes is spooned into the center of the remaining space -- the potatoes do NOT impinge on the plate's edge, they do NOT touch the beef. The spinach is placed alongside the potatoes, so that as it puddles (creamed, remember?) it barely impinge on both the potatoes and the beef. The Yorkshire pudding is portioned and passed, family style. The gravy is served similarly. Or, the Yorkshire pudding is placed on top of the beef, and gravy poured generously over all. Why? It's prime rib, that's why. If the plating is less than perfect, the food is simple, satisfying and deliriously glorious. Don't beat yourself up. Oh, and don't forget a bowl of unsweetened whipped cream flavored with horseradish.
3. (a). A small beef steak and sauteed spinach. Mound the spinach in the center of the plate. Lean the steak against the spinach.
(b) A small beef steak with a pan reduction sauce, and sauteed spinach. Beef covering the center, but not centered. Sauce napping the beef, flowing towards and puddled in the emptiest part of the plate. Spinach, not touching the beef or sauce.
(c) Medium beef steak with mushroom sauce, and sauteed spinach. Steak in the center of the plate. Mushrooms mounded on top. Spinach, wherever.
Note how using different parts of the plate changes focus to accommodating the focus changes between fundamentally similar but slightly different dishes. The steak is always featured, but as the nature of the supporting cast changes, so does the composition.
Every plate tells a story. The story dictates the plating. Keep it simple. One star on the plate, everything else is a supporting player. In a star turn, the star gets the center. In an ensemble, it still dominates. Don't overcrowd it draws attention away. Leave some empty space, it draws attention to the main. Get it?