The weight of the straps and the construction of the bag makes it susceptible to creasing and sagging around the are where the bottom of the outside pocket is.
The only thing you can really do is be careful and lay it on its side as much as you can. Some may argue that this may cause the puffiness to deflate.
You can also send it to spa service for a restructuring/ refurbish. I sent my 5yr old GST, that I carried practically everyday, last fall and it came back just as boxy as the day I bought it. It was flopping over when I brought it in.