I cheat with the breaking-in process for my Part Time, which I imagine would be the same for the City.
I hang my bag from the handles from a hook or door knob.
Then, I put in some really heavy books (quite small - not phone-book-sized books, as you don't want the books to fill the corners of the bag). This makes the bag drop in the base, and soften the handles.
I then add another smallish, hefty bag to the flat part of the long strap (it balances pretty well, if you do it carefully). I make sure the strap hangs directly over the sides of the top corners of the bag. This makes the strap pull on the upper corners of the bag, quickly giving it that slouchy look most of the girls seem to love.
This breaking in process is very, very fast: the heavier the books, the quicker the process.
TIP: make sure your bag is hanging from a hook or door knob that is NOT attached to a door that will be swung open, or the book balanced on your long strap will go flying!