The same way you would clean your glorious hair, with love and affection :)
That and a non scratchy cloth (I think my wife calls them micro fibre cloths) soaked in blessed warm tap water, should do the trick.
If it does not do the trick, then your bag must be dirtier then expected (what have you been up to hmmm?)
To take this extra mess off just try the first suggestion again, but this time add some nothing, because nothing else will get that extra grime off your bag you filthy animal :P
Jokes aside, you could try a non chemical based cleaning agent, but if you're going to use it it will be at your own peril.
Seriously, if warm water and a micro fibre cloth doesn't do it, you could risk damaging your beloved bag, trying anything beyond this suggestion to clean your bag.