This issue is the biggest one for me. We just got back a top last week that had been gone for a year.
DSD's mom just cannot get out of the habit of keeping things (she used to keep all of the clothes before we went to 50/50, and even for a while afterwards). Once CS changed (reduced because of 50/50), she decided to "get back" at us by not sending any clothes for DSD. We wound up buying DSD an entire wardrobe in a weekend.
DF doesn't purchase the clothes (physically - monetarily it comes out of our household budget), so it doesn't bother him that much when things turn up missing. He hasn't put the time in to track down cute stuff on our limited budget. I put a lot of effort into DSD's wardrobe. I research online, pick through clearance racks and consignment stores. DSD looks really cute at our house. It is important to DF that she looks nice (He says so - often), but not important enough sometimes for him to ask her mom where our clothes are.
I would consider labeling things, but I don't know if it would help that much. I am guessing things would lose their resale value if I did that. Plus, I tend to buy socks/tights and things of that nature at Gymboree and basics at Target. So I want the socks back just as much as the t-shirts, and it is darn near impossible to label socks. That was the one piece of clothing that didn't have my name on it when I was in the army.
We used to send her back in the clothes that she came in, but I just cannot stomach sending her to school in the mismatched, ill-fitting, stained, raggedy crap her mom sends her in. She has a lot of clothing at her mom's, just not a lot of decent clothing. We are quality over quantity people, so when we are missing something, it is a big deal.
I don't even want to get started on how DSD's mom doesn't understand what "don't put it in the dryer" means. She has finally (yay!) started sending things back unwashed. Which saves energy, because when she does wash them, I just have to rewash to get the cigarette stink out.
Just writing about this makes my blood boil.