I remember once thinking that I hadnt changed sizes at all. Pants still seemed to fit about the same (this was back in the pre-stretch denim days). Then one day I noticed that the bottom of my jeans was getting all frayed. They were just riding lower than before as my waist and hips shrank, the jeans sunk. Back then the jeans were all high waisted so I didnt get the falling off my butt effect I would get now.
OF course this also worked in the opposite direction..I would fail to notice a weight gain until one day my jeans were...ummm...making me a little too happy when I sat down