Measure over undergarments.  Keep tape measure level and close, not tight.
If possible, have someone else measure you--it's more accurate.

CHEST: Measure at the fullest part of the chest, keeping the measuring tape up under the armpits and over the shoulder blades.

WAIST: Measure around the narrowest part of the natural waistline.

HIPS: Standing with heels together, measure around the fullest part of the body (approximately 8" below natural waistline.)

OUTSEAM/SKIRT LENGTH: Measure 1/2" below the waist (at the waist for items without a waistband), and compare tape measure length to online store item length.