Recruit a friend to come over and take your measurements for you. Make sure you are comfortable with that person seeing you in your underwear. Now strip . . . but only down to your bra and panties. Make sure you are wearing a well-fitting, supportive bra and panties that don't dig into your skin (we all have some of those). This isn't the time for body-shaping underwear -- you're going for accuracy here.
On a piece of paper, write a list of the following words: "waistline," "hips," "high bust," "full bust," "back waist length" and "height."
Tie a strip of elastic around your waist so it is stretched slightly, then let go and allow it to roll into its own position -- this marks your natural waistline. Have your friend measure around your waist at the level of the elastic. Make a note of the measurement next to "waistline" on your piece of paper.
Stand in a position so that your friend can move all the way around you. Have her determine the fullest part of your hips and measure the circumference. Make sure the measuring tape is perfectly level. Note the measurement down next to "hips."
Instruct your friend to take a measurement around the fullest part of your back, under your arms (with your arms hanging down as normal) and above the fullest part of your bust. Again, make sure the tape is level. Note the measurement next to "high bust."
Have your friend take a second bust measurement, this time placing the measuring tape under your arms, across the widest part of your back and across the fullest part of your bust. This is the "full bust" measurement -- note it down.
Tell your friend to measure vertically from the very center of the base of your neck, measuring from a prominent bone at that position down to the elastic around your waist (which should still be in place). Write down the measurement next to "back waist length."
Stand with your back against a wall, making sure you are standing straight up with good posture (even if you don't normally). Ask your friend to place a ruler flat on top of your head so that one end touches the wall, then mark the point where the ruler meets the wall with a pencil. Step away and measure from the pencil mark down to the floor. This is your height. Make a note.
While you are being measured, stand up straight with your shoulders back, and breathe normally. Standing next to a full-length mirror will help you asses your posture.
If your measuring tape is old, make sure it hasn't stretched out or been damaged. If it has, it's time to buy a new one.