I used to think matching sets were a trap, something that looked brilliant on the hanger and then made me look like I was wearing a costume rather than clothes. The fix wasn't a different cut, it was splitting the top from the bottom the second I got it home and trying each piece with something else in my own wardrobe first. If a jacket or a skirt can't survive a weekend away from its other half, it was never a good buy, it was just a matching trick.
The thing to avoid is doubling up on pattern or shine anywhere else on your body. A printed co-ord with a statement bag, a bold belt, or shoes that are trying to say something of their own, and suddenly nobody knows where to look. Same goes for layering another strong colour over the top, a bright coat over a black set is fine, but a patterned one fights the whole outfit before you've left the house. Jewellery wants to go quiet too. These pieces are already doing the talking, so let them.
Quality shows up fastest in whether the two pieces are actually cut from the same fabric, not just a similar shade run through different machines. Hold the top and bottom up to a window and you'll often see one is thinner, which is how you end up with a set that looks matched in the shop and slightly off under any other light. Check the seams on skirt co-ords especially, a lined skirt with a proper waistband will keep its shape, while an unlined one will crease the second you sit down for an hour and never quite recover. Zips should be metal or at least feel solid when you tug them, buttons should sit flush, not proud. A set that's been made properly comes out of a wash looking like a matching pair. A cheap one comes out looking like two things that used to know each other.
Fit is where most of these fall apart, because a set sized to flatter the top often does nothing for the bottom, or the other way round. Buy for whichever half fits you more honestly and have the other taken in or let out, rather than sizing up the whole thing and losing the shape everywhere. Watch the waist on skirt sets particularly, that's where the gap usually shows first, either too snug to sit down in comfortably or loose enough to need adjusting every twenty minutes. A set that's been thrown in a bag for a train journey and still looks like a pair when you pull it out is one worth holding onto for longer than a season.