I have a 7-year old stepdaughter who used to be a SLOB. She never cleaned her room (daddy did it for her before me). But that's not OK in my house!
First we went through her stuff and together we got rid of stuff she didn't need or didn't play with. I let her sell it and earn some extra $$. She had too many things.
We banned food except water in a closed container in her room. I unplugged the cable to her TV.
Then I helped her organize everything into bins with labels. Like one mom said, telling her to "clean her room" was too abstract. She'd get confused because she didn't know what to do first. I needed to help her (but only to tell her what to do, not do it for her). I stand in the middle and direct her. We put on music to make it fun and I set a timer (or else she'd take all day).
I tell her anything that's not put away when the timer goes off gets thrown away. I stand in the middle with a garbage bag so she can throw anything away. Whenever she picks up an object I say "keep or throw away?" If she wants to throw it away she puts it in the bag. If she wants to keep it she has to find a place for it. We do this every night as her before bed routine.
When the timer goes off I walk around and start to pick up objects not put away. I move slowly because she'll often grab it and put it away (I let her do this).
She's commented herself that she loves it when her room is clean so she can relax! I also try to keep the house clean so I'm setting a good example. It's hard to tell her to clean her room when I have junk lying around the house too.
We also have the House Fairy come (www.housefairy.org) and she leaves a surprise (a $1.00 toy) if her room is clean. But we haven't needed that since we made cleaning it part of her routine and your daughter is probably too old to believe in the House Fairy.
The funny thing is my stepdaughter's room is VERY CLEAN at our house and we don't have to yell or punish. It's totally trashed at her mom's house and they are always yelling at her, grounding her and such there for it.
She even vacuums and dusts her room at our house, I'm sure she doesn't do that at her mom's.
It's not always the child, but how you help them and raise them. At our house we make a big deal about how she's contributing to our family. We have even shown her how her doing her jobs help us, she does all her chores at our house with a smile and we reward her with hugs and smiles and we play family games.