A slingback is a type of shoe, usually heeled, that has a strap that wraps around the heel instead of a full heel covering. This strap can help make your shoe stay on better than a backless shoe. However, if you get an ill-fitting, too-tight strap, it can cause blisters. If you have a perfect fit but don't take care of your slingbacks, the strap can stretch, resulting in reduced support or the annoyance of a strap that keeps falling down. With a bit of effort and the right care, you can keep your slingbacks in good, comfortable shape for years.
Purchase slingbacks with straps that fit snugly without pressing into your heel. Shop in the afternoon when your feet have swollen a bit. Try on the shoes and walk around to make sure the back doesn't slide off or rub on your heel.
Rest your slingbacks at least one day between wearings. This allows the leather or other material that the strap is made of to retract, extending its "shelf-life" before it stretches permanently and loses its wearability.
Store your slingbacks in a cool area that's not too dry, particularly if they're made from leather, as hot temperatures and dry air can cause the materials to become hard and even crack. You want the back of the slingback to remain as supple as possible, so it conforms to your foot without cutting into it.
Slip your slingbacks onto a foot form on the days you're not wearing them to help the back strap keep its shape. If the shoe came with a plastic heel piece, use that. If not, you can either purchase a boot shaper or stuff the heel with socks, which can also help to absorb excess moisture (as in your foot sweat) from the shoe. Never hang your shoes from the back straps, as this will eventually cause them to stretch out.
To get the longest life out of your slingbacks, look for styles with adjustable back straps. This way, you can control their fit as the straps stretch out over time or become too tight after a day of walking.