Yes, Clinique and Calvin Klein are great...if you have money. For the rest of us, off-brand products will have to do. The good news is that they can make you look great if you know what to buy and where to buy it.

Ignore trends! They change almost by the hour so it will be costly, not to mention exhausting to try to keep up. Instead, focus on defining your own individual style. Being unique is hotter than being a carbon copy of a fashion magazine anyway. Only wear the trends that look good on you--if you're long-waisted with a soft tummy, don't wear low-waisted pants. Instead, go for a pencil skirt--classic, and so always in fashion, plus (bonus!) you get the illusion of a perfectly proportioned figure. A mix of trendy pieces and classics always looks classier and more elegant than an entire wardrobe of fashion-magazine items. Go to the drugstore for beauty and hygiene products. The options there are greater than that of most supermarkets, and cheaper than that of most specialty stores.Be thrifty with hair products. If you must splurge, do so on conditioner, not shampoo. Shampoo is for cleaning the hair and unless you swim a lot, you won't need anything special. (If you do swim, you can use a clarifying shampoo or just wash your hair twice.) Never use any more shampoo or conditioner than a dime-sized amount unless you have long hair. And be sure to also cut off split ends.
Check out thrift stores. They can be a huge, mostly untapped resource. Go to ones in college towns where young people move often and may get rid of clothing only a year or two old. Also look for outlet stores. Again, even though the clothing is cheap, look for things that look great on you, not just things that "fit ok."

Go for the cheap jeans. Others will never see a difference between a $25 pair and a $125 pair. In fact, many designer jeans are overwashed to make them soft, and so they wear out much faster; hence, a cheaper pair may fit as well and last longer too. However, be aware that sometimes you do get what you pay for: at least some of the price of designer jeans reflects the quality of the cut, the stitching and the fabric. Don't worry about where you shop - the important thing is whether the item fits, is flattering and a good color. Even low-priced bargain stores (even Wal-mart!) can accidentally make something good, so always have an eye out.
Shop for off-season clothing. Look for end of the season clearance sales. Also look for before and after Christmas sales but be aware that, even at a deep discount, some clothes may not be worth the price. Know ahead of time how much you intend to spend on something and try to stick with it. Look for versatility. For example, look for a button down shirt that looks good open over a t-shirt or buttoned under a sweater. Extra points if it also looks good tucked into dress pants or a skirt for fancy occasions! Make sure your new clothes are machine washable. Trips to the dry-cleaners can eat up money for gas and service, and no one likes doing their laundry by hand.

Go to a discount chain for your trendy shoes - you can get cheap, fun stuff there, because you don't need it to last more than a season or two. But for basics you'll wear every season (basic pumps, black boots that you can wear and look good in the winter, simple strappy sandals), buy good-to-excellent models that are seriously comfortable, in real leather rather than fake, and buy the oils and polishes too so you can keep them in good condition. Try remnant department stores like TJ Maxx for good-quality shoes at discount prices. Stick with the basics for makeup: mascara, lipgloss, blush, concealer, and maybe some liner and shadow. Especially if you have problems with acne or dry skin, a good-quality foundation for sensitive skin (applied with a very light hand on the sponge) is a worthy place to splurge, and really contributes to a polished look. Bad foundation makes the prettiest woman look trashy and tired, and good foundation lifts your whole look. Use disposable razors and men's shaving cream. A pack of disposable razors can last longer and men's shaving cream is much cheaper than women's lotion and gels.
Have a black, a brown, and some summery shade (white is classic, but a pale gray or a subtle metallic can actually be more versatile) of purse--they don't need to match your shoes.

As terrible as Goodwill and the Salvation Army are made out to be, they are a terrific places for cheaper clothing, even compared to thrift stores.At thrift stores, if the purse you HAVE TO HAVE is there, but it has a rip in it, offer a lower price on it. Generally, they'll let you have it to make you happy, so you'll come back, or even just to make the sale. Most clothing is slightly marked up from value, even at thrift stores. Don't be afraid to bargain. Use what you already have. A $7 bottle of shampoo is cheaper than a $3 bottle of shampoo if you already own it. Buy what's on sale. Variety is the spice of life, and it's been proven that using different products makes hair healthy.

Search the web to find out what products are available. Just because it's not heavily advertised doesn't mean it's not good. Try not to buy clothing unless you actually need it. Just because something is on sale doesn't mean you're saving money unless you would have bought it anyway.
If you can sew (and even if you can't) don't be afraid to alter clothing. If you have an old pair of jeans that are beat up on the bottom but the waist still fits then rip out the inseam and make a mini skirt. Got a pair of pants that are too long? Hem them yourself. If you can't sew use those iron on strips or (for a temporary fix) use duct tape to tape them up on the inside.Consider forgoing a gym membership and work out at home instead. Unless you're very serious, some dumbbells, exercise instructions off the internet, and a jog around the block are just as good.

Many low cost and even designer personal care products (shampoos, moisturizers, etc.) contain potentially harmful toxins. If you're concerned about what chemicals get absorbed into your skin, you'd be surprised how many cancer causing chemicals we dump into our systems in the name of heath and personal care.
If you're obsessed with your reputation, NEVER go out telling people you just got new jeans half-price at Wal-Mart. Unless you tell someone, or they look totally Wal-Marty, nobody ever has to know...