i've has my bag for a few weeks now and it's not dirty at all, which is suprising since I walk around highschool with it all day. I guess you just have to be careful with it.
Sometimes some lint will get on it and then i just use a lint roller. As for stains the SA told me for all my bags(except the leather parts) to use unscented baby wipes to remove stains. I've used the wipes on my sig.khaki small hobo and they worked great and left no stain at all.