Dance belts, tights, and leotards all have a high spandex content. Spandex (aka Lycra if it comes from Dupont) loses its stretch and is destroyed by hot water. It can also be damaged by sweat and skin oils, so don't let your dirty damp dance clothes fester in an airtight dance bag.
Cold water is the key to keeping your dancewear happy in the laundry.
It's OK to use the washing machine set to cold, on the 'delicate' cycle.
Hang everything to dry.
Never put dance clothes in the dryer.
If you live in a humid climate and it seems like your stuff takes forever to dry, wrap the clothes in a towel to remove excess water prior to hanging.