My 10 y.o stepdaughter thinks shes an adult! I need some advice about dealing with her behaviour.?

She stays with us every fortnight but her attitude and behaviour is starting to wear thin. She swears like a trooper, does whatever she feels like, shows no respect for our home (draws on walls etc.) but the biggest problem is she thinks she knows everything!!! Her mother has always bought her loads of expensive things (she comes back from the shops with a Nintendo DS when most kids would get a pack of lollies) and as such she doesn't appreciate or care for anything. Also, her mother never 'censors' anything so shes always been around very adult conversations (swearing, dirty jokes, general b#tching) and situations like shes one of her 'mates' which I think is why shes so headstrong and opinionated. I don't mean she can't have a say in things but there are limits and I sometimes have to remind her that shes a child and she doesn't understand everything. I wish I could give some examples of what I mean but I wouldn't know where to start!! I guess what I need to know is what are some good basic rules to lay down and how do we enforce them? We have tried to take something away from her for the weekend if she's been really but it doesn't work because most of her stuff is at home and she knows shes only here for a short time so she has the attitude of "stuff you- I'll do what I want cos I'll be going home tomorrow anyway". Underneath it she can be such a lovely girl but its very hard to teach her what is acceptable when she is only with us for such short periods of time, then goes home and gets corrupted!!!