Here's what I did...
When she is out/at school, have a really good clear out of her room. Allow her the bare minimum, such as one (or 2...or 3) bedtime toy(s), essential clothes (school uniform and other ordinary clothes for next few evenings/weekends) hair stuff, a few reading books and other essentials. ALL the rest of it, go through and decide what you would like to see the back of and put in boxes/black bags. Then, put the rest (clothes/toys) into either various carrier bags and put into loft, or (jewellery, DS lite, other books, toys etc) pack carefully into various boxes and remember what's in them, and store out of reach somewhere in your room/kitchen/out of the way.
Buy her something new to put into her room. Something small and not valuable which would have an obvious place to be put away - hair slides, a book, a new tee shirt.
When your DD returns, before she goes to her room, sit her down with a drink and snack and be calm and gentle. Explain to her that her room has been untidy for long enough and you need her to learn to keep it tidy herself, so you can get in and clean it when you do the rest of the house. Let her know that , to make it easier for her, it has been cleared and cleaned so that she has less to look after, give her an encouraging hug and give her the gift to put away carefully. Go to her room with her and judge her mood. If she is happy, tell her straight and calmly, if she is terribly upset, you could invent some fairies who helped you and who might bring back some of her things if she can show she can look after what she has in her room already. Do not allow her anything to play with that you have removed on the first night. The next day, keep tabs on what she does and micromanage what she needs to be doing ...e.g. putting that clean skirt on a hanger, putting the dirty clothes in the wash box, make bed (show her how and make it fun, it isn't obvious to children) put book on shelf and hair things in their place. pick up rubbish and put it in a bin. Let her know that when you see her doing these things without being told what to do, she can start having some of the things back in her room.
When she has started to get the hang of something and you see she has remembered to do something without being reminded, make a huge deal and praise her, and when she isn't looking, the fairies can bring a box/carrier back to her room to put away. Explain now the fairies think she is doing so well, she is able to look after even more, so this bag has to be tidy too.
The trick is, don't stuff her room full of so much stuff, there isn't time in the day to look after it. I didn't give my DD everything back, the things I wanted to get rid of stayed in the loft and the fairies gave it to charity when we moved house. (I told her it was me, but in hindsight I wish I'd invented the fairies, it would have been kinder). Her room does still get in a mess, but she will give it a thorough tidy and clean when she's given time and told she must. She can also keep it tidy over several weeks at a time, so I have to keep an eye on it and give lots of praise when that is happening too.