Birkin owners, Do you feel the need to tug at the straps to tighten up your Birkin when it is in use? I often find the straps loosening up too much, even though they are resting on the strap guides. If I don't do anything about it, I find that the bag gets untidy, like the way celebrities use their Birkins, and I don't like it a bit. And this means tidying up the bag every now and then, which is rather irritating. :s Any tips?