Measure four numbers, not one. Bust, underbust, waist and hip. Letter sizes between brands do not agree with each other and never have. Four numbers on your phone solves it permanently.
The underband does the work, not the straps. If a piece is riding up at the back, the band is too big, not the straps too loose. Tightening straps to fix a loose band is the most common reason lingerie ends up uncomfortable.
Adjustable and lace-up beats fixed. Lace-up backs, tie sides and slider straps fit a much wider range of torso lengths, and they keep fitting through a weight change instead of becoming a wasted purchase.
Steel boning on anything structured. Plastic boning bends out of shape within a couple of hours and then holds nothing. On a corset or bustier it is the whole difference between the piece working and not.
Stretch lace forgives, woven satin does not. Satin has no give, so the measurements need to be right. Stretch lace and mesh have a real range, which makes them the safer buy when you are between sizes or ordering a brand you have not tried.
Hand wash, always. Lace, elastic and underwire all die in a machine, and a dryer finishes the job. Cold water, gentle soap, lay flat. This is the difference between two seasons and five.
