For such a small garment, bikinis go are under a lot of wear, tear and stretch. Between the sun, chlorine, direct sunlight, salt water and constant wet to dry, it is important to take proper care of your suit.
Keeping your suit tight and perky, and fitting comfortably, prompt washing is critical. Caring for your swimsuits is easy and should be done by hand. If washing in a machine is necessary, be sure to use a mesh bag for intimates, mild detergent and a delicate wash cycle.
TLDR: The best way to wash your suit is in your post activity shower.
4 Easy Steps to Care for Your Bikini
Rinse as much sand and residue as you can under cool water to remove any body oils, salt, sand, or lotions.
Make your suit a bath of cool tap water and a teaspoon of laundry detergent. Gently scrub high wear areas then soak the suit in its bath for 20 minutes. Rinse well.
After all the soap has been rinsed out, gently squeeze out excess water. Be careful not to wring it to avoid any stretching. Lay your suit flat to dry in a spot away from direct sunlight.
Store the suit once it is completely dry to avoid mildew.