Wash knit tights by hand. Subjecting them to the full force of the week's laundry pulls and stretches the tension of the knit, and they will emerge several yards long.
Use a liquid soap and warm water, then immerse the tights and let them soak for five minutes. Rub the toes and heels gently so as not to stretch the knit, soak and squeeze the soapy water out a few times. Rinse in warm water.
Rinse two or three times to get rid of all the detergent, then blot the tights with a towel. Hang them in the fresh air or put them in the dryer so they regain their shape quickly.
Use a gentle machine wash program that doesn't pound or spin your tights to death. A wool wash setting agitates them gently in a soap solution, rinses them well and spins slowly without stretching.
Wash one or two other items with your tights to avoid wasting energy on just one garment, such as a knit sweater or scarf.
Take the tights out of the machine and blot them thoroughly with a towel, making sure not to twist or stretch the towel with the tights inside it.
Put your tights into a mesh bag that you can buy at your supermarket in the laundry section. These mesh bags are made for washing small items in a machine without them getting wrapped around other laundry and being pulled out of shape.
Set the temperature at less than 40 degrees. Anything hotter might still damage the weave of your knit tights and weaken the tension.
Remove the tights from the mesh bag when the cycle is complete and blot them with a towel to get as much moisture out as possible. Slip them back into the mesh bag for a short time in the dryer or hang them where a moving current of air will dry them quickly.