Grab a flexible tape measure (these are dirt cheap at fabric stores!) and a small metal binder clip.
Say you want to know where a skirt that measures "21" from natural waist" will hit you. At the knee? Above the knee? Just hold the 0 end at your natural waist and let the tape measure hang. The binder clip provides just enough weight for the tape to hang straight and the little arms show you right where the 21" will be. And because the tape is hanging, like the skirt will, you get an accurate measurement over your stomach and hips.