Prevent clothes from getting stretched?

Check the labels of each clothing pieces you own, there are correct instruction on how to keep them clean & keep it in good state. Follow the instruction and you should be fine.

Hand wash, low temperature, dry clean...there are many ways to clean them. I usually check the labels before I buy them so I can try to have similar kind of materials so things gets easier.

You can buy a laundry bag
Its a net bag that stops it from getting pulled about in the wash