Steel boning, not plastic. On a corset or bustier, plastic boning bends out of shape within a few hours and stops supporting anything. Steel holds all day and all night. It is the single biggest difference between a corset that works and one you take off at 4pm.
Shop bottoms by rise, not just by size. A high-waist short that sits at the natural waist stays there. A mid-rise on the same body works its way down every hour, and you spend the night fixing it.
Adjustable is worth paying for. Fixed straps are cut for one torso length. Sliders, lace-up backs and tie sides fit a far wider range, and they let one piece work across a weight change.
Vinyl and holographic run smaller than mesh. Coated fabrics have almost no give. Size up on anything vinyl, patent or holographic, and stay true to size on mesh, stretch lace and jersey.
Accessories carry an outfit. Wings, pasties and hair pieces are one size, cost the least, and change a look completely. If the clothing budget is tight, spend it here.
Wash cold and hang dry. Heat destroys holographic coating, elastic and sequins in one cycle. No dryer, ever, no matter what the tag says.