A swimsuit that fits correctly feels slightly too tight when it is dry. Swim fabric relaxes when wet, so the suit that feels perfect in the changing room is usually the one that goes loose and see-through in the water. The test I use is simple: pull the leg opening about an inch from your skin, and if it goes further than that, size down. Straps should sit flat without digging, and a one piece should let you raise both arms without the crotch riding up. Below, the one pieces worth sizing carefully, the tops that genuinely hold, and the bottoms where coverage is the whole decision. Sizing up is the instinct and it is almost always wrong. A larger swimsuit has more fabric to relax, so it goes looser faster and the leg openings gape once wet, which is the exact failure people are trying to avoid.

One pieces are the hardest to fit because torso length matters as much as your size, and almost nobody lists it. If you are long through the body, look for anything described as long torso or adjustable at the shoulder, or you will spend the day tugging. Torso length is the measurement to know, taken from the top of your shoulder, down through the crotch and back up. If a suit sits comfortably on land but pulls when you sit, the torso is short for you regardless of what the size label says. Ruching through the middle buys you a few centimetres of adjustment, and a suit with adjustable straps buys you rather more.

A bikini top only holds if the band does the work. Underwire and thicker bands support far better than a triangle with a string, and a halter shifts the load to your neck, which is fine for an hour and tiring for a day. Band structure is the thing to look at rather than cup shape. A wide elastic band under the bust, an underwire, or a proper tie that pulls from the sides will all hold; a thin string that ties at the back will not, whatever the cup looks like on the hanger. If you are between sizes on a bikini top, take the smaller band and the larger cup rather than sizing the whole thing up.

Bottoms come down to how much you want to think about them. Higher waists stay put through waves, cheeky cuts do not, and a tie side gives you a size and a half of adjustment for free. Coverage is worth being honest about, because the bottom you can forget about is the one you will wear. High waists and full coverage cuts stay put through waves; cheeky and thong cuts move and need adjusting. A tie side gives you real adjustment as the fabric relaxes, which is why they last better than a fixed hip.

Monday Swimwear cuts for real torsos, with a long torso option on much of the line and enough band structure that the tops still hold after a season. This is the label I recommend to anyone who has given up on one pieces. Sizing runs true and the long torso option is a genuine second pattern rather than a longer strap, which is unusual at any price. If you have never found a one piece that fits through the body, this is the most likely place to solve it.

And the cover ups, because getting from the water to lunch is the other half of this. The cover ups that go with all of it, from sarongs to kaftans, so you can get from the water to a table without a full change. These are cut generously enough to go on while you are still damp, which is the only test that matters for a cover up.

Rinse in cold water the moment you are out, every time. Chlorine and sun are what kill elastane, and a suit that gets rinsed lasts two or three summers instead of one. Never wring it out and never leave it balled up in a bag; the creases set into the fabric permanently. I hang mine over a chair back in the shade, which is unglamorous and works better than anything else I have tried.