I really think that you will get more answers posting on the child behavior forum -
http://www.medhelp.org/forums/Child-Behavior/show/64
Its a fairly common problem that I see posted there. The one piece of advice that I can give is that rewards at this age really don't work to well. Kids of this age can't deal in future events. I think it is more about getting into an established routine.
Having said this I wonder why you posted on the ADHD forum. If you also suspect your child may have AD/HD, then I should mention that in the book, "The ADD/ ADHD Answer book," by Susan Ashley, on page 72 she does talk about the problem of Encopresis (or wetting the bed or clothing). If you also suspect your daughter might have ADHD, then I suggest you buy this book. Best wishes.