C.,
Jamie has some great ideas. I would like to add that you can also charge them for not having a clean room (they must pay you or have allowance reduced.) Children that age like to leave the house and do things with friends or have them come over. You can use that as leverage also.
The bottom line is that if you are inconsistent or do it for them or let their rooms go messy, they will stay messy. The rooms don't have to be spotless, just picked up and clean enough to walk through and find everything.
Also, make sure there is a place for everything and they know where it goes. A very messy room can be overwhelming to anyone, especially children. So teach them how to take it one corner or one project at a time and do it WITH them for a while. Saying "go clean your room" will not get results. I remember my mother nagging on me about this and it never inspired me to clean, especially because her room was so much worse than mine.
You can even create a chart of what must be done because "clean" can be a very subjective description, so be specific. (Laundry in basket, chair pushed in, bed made, belongings in correct bin, floor picked up and vacuumed, etc.)
Best of luck,
S.