Heat and fabric softener are what kill stretch. Elastane — the Lycra or spandex in leggings, stretch denim, jersey and swimwear — is degraded by heat, by fabric softener and, most of all, by the tumble dryer. Keep it cool and let it air dry and the stretch lasts far longer.
Wash cool and gentle. Wash on a cool cycle up to 30°C on gentle, or hand wash — swimwear is best hand washed. Use a mild detergent and no bleach. Never add fabric softener: it leaves a waxy coating that clogs the fibres and impairs how the garment stretches and recovers.
Air dry — never tumble. The tumble dryer is the single biggest destroyer of stretch, so lay these pieces flat to dry in the shade rather than putting them through any heat.
Rinse swimwear straight away. After the pool or the sea, rinse swimwear immediately in cold, clean water to flush out chlorine, salt, sunscreen and body oils. Don't leave it rolled up in a wet towel, and dry it flat in the shade.