Buy a thong that’s one size larger than your regular undies. So if you normally wear size small underwear, get a medium thong; if you wear medium undies, get a large thong; and if you wear XXL underwear, do the world a favor and avoid thongs altogether.
When trying on a thong at the intimate-apparel store, you’ll be required to put it on over your existing underwear (or over your long johns if you live in Wisconsin). Do this in the dressing room, not the store aisle.
You’ll need some time to get used to wearing a thong. Some women feel as if they’ve got a permanent wedgie. Others start putting on a thong only to discover that they never took off the previous one, which is wedged in pretty tight.