The best way is to find a freckle and use it every time.
LOL! That's what I do. For record keeping, what matters most is consistency.
As for measuring for clothing, I have no idea. I'm bad about the waist and the hips. Proper "hip" measurement for me seems more like my butt, not my hips. My hip bones are a lot higher than where I think my hips must actually be.
Dr. Oz wants us to have a waist size of 32 or smaller and he allows for sucking in. I do not suck in, but that's just me. And I like to pull the tape pretty tight only because that way I can't second guess the way I do it each time. If I'm always pulling really tight then it's the same every time.