If you're going to track measurements here are the guidelines:
Bust: Measure around the fullest part of your bust with your bra on (don't use a padded bra! :)
Waist: Measure around the narrowest part of your waist. This should be on the same level as your navel. If you are one of those poor people who don't narrow at your waist, simply measure at navel level, keeping the tape level all the way around.
Hips: Measure around the fullest part of your hips. This should be about 10 inches below your waist line or just about where the bottom of your jeans zipper is.
You'll get more accurate measurements if you have some one else take them for you. If that's not possible, or if you're too embarassed, position the tape so that it fits snugly, but not tight. place your finger on the tape where it overlaps, then read the number once you let the tape drop. If you try to bend over and look you'll distort the measurement. This is the way measurements are taken for dress-making and fitting formal wear.
Hope this helps!