You can spot a good pendant from across a room by how it sits, flat and settled against the chest, catching light in one clean point rather than swinging about or twisting side on. A poor one tells on itself straight away, the chain too thin for the weight of the charm, so it flips over and you spend the whole day turning it back round, or it dangles at an odd angle because the jump ring was never quite right.
Getting the length right matters more than people admit when they're buying. A pendant that sits at the collarbone reads differently to one that drops to the breastbone, and it changes with every neckline you own, so think about what you actually wear before you decide. Too short and it competes with a crew neck, sitting awkwardly on top of the fabric instead of resting against skin. Too long and it can disappear under layers or catch on a seatbelt, which is a small daily annoyance nobody mentions until it happens to them. Watch the clasp too, a fiddly lobster clasp on a chain you need two hands and a mirror for gets left in a drawer no matter how much you liked it in the shop.
Pendants don't like company. One strong piece on a chain is usually the whole outfit's statement, so keep everything else quiet, a plain jumper, a round neck top, a shirt worn open rather than buttoned to the throat. Stacking another necklace alongside it, or wearing it with a top that already has a busy neckline, a frill, a deep V with embellishment, just muddies the point of having it. It also struggles against scarves and high necklines generally, the chain gets lost and the pendant ends up pressed flat against fabric with nowhere to sit.
Over months, the chain is usually the first thing to show wear, especially if you sleep in it or wear it in the shower, so take it off for both. Plating can thin at the clasp and where it rubs against a coat zip, so wipe it down after wearing and keep it away from perfume sprayed directly onto skin. Stored loose in a drawer it will tangle itself into knots within a fortnight, a small pouch or a hook solves that completely.
Where it falls short is anything high necked or heavily layered, it simply has nowhere to go and ends up hidden rather than worn. For everything else, a pendant on a good chain is one of the easiest pieces of jewellery to get right.