The Baboon who posted above mentioned Paula Begoun, thus demonstrating that this baboon is considerably smarter than most of its kind. Paula has written several books (“The Beauty Bible”, “Don’t Go To The Cosmetics Counter Without Me”, something similar about hair products . . . ) that both explain (in a nonscientific way; she’s a journalist, not a chemist) formulations of cosmetics and review individual ones. The reviews are brief, and focus on problem ingredients, but they’re fairly helpful. She’s got a website up at http://www.cosmeticscop.com/ - it’s a nice resource. I have several of her books, and they’re quite fascinating (to me) and pretty solidly informative.
You asked about shampoo, though. Shampoo (and hair products in general) are surprisingly similar. There just aren’t that many different chemicals that the formulations use. There are a few problematic detergents, that will tend to be harsher to the hair or more irritating to the scalp, and those are to be avoided, but there’s nothing that sets salon hair products apart from the drug store options. Personal preference is a lot of it.
Skin care products, again, can’t be judged on their prices, but their formulations differ a lot more. There are all sorts of facial cleansers that won’t cleanse your skin - or will irritate it (and promote breakouts). Plenty of others have strong scents that can cause allergies or do harm to you. Again, the site above sells Paula’s line of skin care products. (Her motives seem fairly pure: her books don’t recommend her stuff to the exclusion of all else; my impression is that she wants to earn some money while selling decent, well-formulated products.) They’re all unfragranced, and I’ve used her oily-skin cleanser for years. I’ve never found a product to equal it. The products are also very reasonably priced - a little cheaper, in fact, than the average drugstore brand.
But it’s hard to make any sweeping statements. Stay away from anything that makes your skin tingle (they tend to be irritating) and anything with strong scents (same thing, plus the possibility of an allergic reaction). Higher priced products may be virtually identical to drugstore brands - or even worse. Most of the drugstore brands are owned by the same parent companies as the higher-end brands, and many of them sell virtually identical products.
Personally, I stay away from salon brands, any kind of soap - as opposed to a detergent cleanser, anything for my face with any fragrance whatsoever, and anything with annoying TV commercials. That’s about all I can suggest without getting into all sorts of specifics. You might check to see if Ms. Begoun’s books are available at your library. Page through the informational front section, and check the reviews of specific products. They helped me get my face reasonably smooth, at any rate.
StarvingButStrong’s pretty much right here, though. There are a few very bad products, and if you have problematic skin you should do some real research, but in general, if you’ve got ok skin and hair, there’s no product that’s gonna make you look amazingly better, and very few that’ll hurt you. There aren’t any silver bullets; all the products out there are mixed from the same set of chemicals, mostly from the same suppliers, and there’s just not all that much difference between 'em.