Stabilize with cardboard bottom and medium Purseket in 30. I don't like too much sag and even with all this it sags a wee bit which actually looks cool.
This was a big question for me before I bought my Speedy 30... I actually found the sagging is not bad... so I don't use anything. The more stuff you put in it the less it sags... and the bigger my bag the more stuff I find to put in it... so maybe that is why mines doesn't sag so much
I don't mind the sag, I think it's very chic. None of the celebs who carry the speedy seem to mind sag either. The bag was meant to look like that or they would have reinforced the canvas to add structure back in. But usually with the amount of stuff I carrry, the sag is very minimal. I have a 30 though, I'm sure you encounter greater sag problems with the 35 and up.