What you wear in St Barths is decided by the clock, not the occasion. The island runs on two speeds: a long, salt-and-sunscreen daytime that asks almost nothing of you, and an evening in Gustavia that quietly asks for quite a lot. Saint-Barthelemy is French, and the dress code everywhere is informal, with smart casual understood the moment you sit down to dinner. Nobody will turn you away for wearing a sandal. Everybody will notice which sandal. So pack in pairs. One swimsuit good enough to be the whole outfit, one hat that survives being sat on, one bag that can hold a book and a bottle of water and still look like it was chosen. Then a second set for after six: something in silk or satin, and a heel, because the restaurants here are worth changing for. That is the whole logic of this edit. It moves through the day four times, alternating between what works at noon and what works at nine, so you can see how a single suitcase actually functions once it lands.

Beach club hours are the easiest part of the day to dress and the easiest to get wrong. Heat and salt water are hard on clothes, so the pieces that earn their place are the ones built for it. A one-piece in a dense, high-recovery knit holds its shape after a swim in a way a flimsy bikini does not. Straw and raffia want to be woven tightly enough to keep a brim, because a soft hat is a wet hat by lunchtime. Rattan and basket bags take sand and shake it out. Sizing is where swimwear goes sideways. Most designer one-pieces run long in the torso and narrow in the leg, so if you are between sizes, go up and let the fabric do the work. Try the hat on with your hair the way you actually wear it on holiday, damp and pushed back, not blow-dried. And take the logo question seriously. One piece with a house mark in the room is confident. Three is a uniform. Pick the swimsuit or the bag, not both.

Dinner in Gustavia is the one moment the island stops being casual, and the trick is that you do not go back to your room and rebuild yourself from scratch. You change two things. The dress and the shoe. Everything else, the hair, the tan, the jewellery you put on at breakfast, comes with you. This is why satin and lame travel so well. They read as evening under warm light with no styling at all, they pack down to nothing, and they do not need pressing after a flight the way linen and cotton do. A slip dress and a maxi in a fluid silk will each get worn twice in a week and look different both times. Heels are appropriate here and worth bringing. Gustavia is a harbour town with real pavements, not a beach, so a slingback or a pump is entirely wearable. Pick a heel you can stand in for three hours, which for most people means a shape with a wider base or a lower, sculpted heel rather than a needle-thin stiletto.

Lunch is where St Barths dressing is actually decided, and it is harder than dinner. You need a dress that works over a swimsuit, in a car, on a terrace and in a photograph, and it has to do that at the hottest hour of the day. Printed cotton, silk twill and a light jersey all clear that bar. A midi or a maxi in a print is the most useful thing you can own here, because it hides sunscreen, moves in wind and needs no accessory to feel finished. Flats are the point. A leather thong, a fisherman sandal or a plain flat sandal will carry you from sand to a table without the wobble, and they are the shoes that get worn every single day of a trip. If you buy one thing new for the holiday, make it these. For the bag, go bigger than feels necessary. A medium tote in smooth leather or a woven raffia crossbody holds the sunscreen, the wet swimsuit and the wallet, which is the real brief.

The mistake we see most often is over-packing the evening and under-packing the middle of the day. You will wear a printed midi and a flat sandal five times. You will wear the most embellished thing you brought once, if at all, and it will spend the week in a garment bag. Build the suitcase around lunch and let dinner be two good pieces. We would also go easy on the black. It photographs heavy in this light and it holds heat, so keep it to one sharp piece after dark and let cream, raffia, a strong print or one saturated colour do the daytime. And be honest with yourself about the heel height. A 130mm platform looks magnificent in a product shot and less magnificent on a harbour front at midnight. If you take one thing away, buy the flat sandal before you buy the dress. It is the piece that touches every single day of the trip, and it is the one people always leave until last.