Always return clothing and belongings to the other parent. Make sure the clothing is properly washed and folded. Also, be understanding if clothing is not returned immediately. If the other party is putting the clothes through the wash, it may not come back right away. Furthermore, if you demand the clothing back immediately, do not be irritated if it comes back dirty.
Take care of the other parent's toys/belongings. There is no need to stop little Johnny from taking his toy truck to Mommy's house. That puts the child in the middle. Mommy needs to be adult enough to return the item. Be reasonable!