What I can't figure out is they are always talking about "clean lines" and then show the "victim" a suit with the blouse hanging out from the bottom of the jacket. I was taught that this just made you look like a slob.
Or getting a woman to buy tops that make "everybody" look like they are wearing maternity tops
Please forgive the long post, as it isn't too much of a reply to your comment, but more of me rambling on about how much better my clothes fit now that I know what to look for!
If the blouse-out-of-jacket is done well (and that's kind of hard to do), it does offer length to your body, which makes people look taller and/or leaner. It most likely doesn't work for a formal power-suit with a closed jacket, but it will work for a more casual setting, with the jacket open and paired with a great long necklace. "Clean lines" is basically not wearing things that draw lines in places like love handles/belly or at the ankle if you have short legs. Putting the outfit together so that one piece flows "cleanly" into the next (blouse into jacket, jacket into slacks, slacks at the proper length and tailoring) really does wonder for your figure, even for an Oompa Loompa like me.
I'm short, so I keep my pants a little long and not cut them where they meet my shoes. It's hard to believe, but if you do it both ways, you'll see that it really does make you look taller if your pants cover your shoes a little bit (not dragging or bunching). I also avoid cutting things right at my belly, since I have a short waist. The newest style is a longer shirt (thank goodness belly shirts and low-rises are going out of style), so I wear those so they don't cut at my "problem area," which is the spot right around my navel. It's all about drawing the eye away from parts you'd rather not attract attention to and not creating any bulges with the fit of your clothing. Blazers should make a waist by having curved seams. Look at the back of the jacket, if the seams are straight, you will make a square shape out of your body, which isn't too flattering. The hem of the blazer should hit a little past the top of your hips, flowing nicely into your pants. With just the right cut in both blazer and pants, you can definitely create a leaner look around the hip, which is what I need with my big rear.
The best thing to do is to try on badly-cut clothing versus well-cut clothing. A beautifully cut blazer can make a huge difference and will last forever. I didn't believe that until my best friend threw one on me. The best example of when it didn't work was when she had me try on a brocade, bolero-cut jacket. The brocade was really heavy, so it overwhelmed my smaller top. The bolero-cut ended right around my navel, making me look shorter. However, when my best friend (5'11" and very voluptuous) tried it on, it looked sensational on her. She's a bigger woman and a heavy fabric compliments her size. Her height made the bolero a good choice.
As for the empire-waisted tops... they do look like maternity tops, but I love them. For one thing, they give the illusion that my bust is a little more proportional by drawing the line right underneath them. They hide my belly well, and as long as I don't wear them too big or too blousy, I don't look pregnant. This is definitely where cut and the quality of the top are important. If it's sewn well at the empire waist, then it will hang just right. They tend to work better on small-top women. Also, the top should not end right at your hips, but should be slightly longer. Otherwise, it'll look like you're wearing a lampshade. If you follow the line from shoulder to foot, the top should curve around my bust, then flow down into my hips and into my jeans without any lumps or dips.
This even applies to jewelry. My friend can't wear chunky jewelry, despite her height and size, because she's naturally very fine-boned. She'll wear finer styles like bangles or light chains and thick bracelets really do look odd on her. However, I'm naturally big-boned, so even though I'm smaller in overall size, I can wear big beads and thick bracelets easily. Length of necklaces are also considered. I have to wear really long or almost choker-style ones, because of my short neck and short stature. Necklaces that fall right at the breastbone don't work for me. Nor do pendants, because they look like I'm wearing a sandwich board.
It's hard to describe, but creating "lines" and watching your hems and creases is the only thing that makes sense about that show. I'm sorry this is such a long post, but ever since my best friend introduced me to the idea of creating lines and essentially fooling the eye and hiding problem areas, I've been an avid student on how my clothes fit. It won't make me buy trendy stuff, but I'm a lot more careful about buying just anything. If it didn't really make such a difference, I wouldn't care, but it does do wonders for how your clothes look and it makes you think more about "does it look good on ME?" versus "does it look good?"
I'm still pretty amazed at how little details make a huge difference when it comes to that.