Take your bra off and measure around the fullest point of your bust. Make sure to go under your arms, not around them :P
Measure around the smallest part of your waistline with one forefinger between your body and the measuring tape.
Stand with your heels together and measure around the fullest part of your hips.
Stand with your heels together, keeping tape straight and perpendicular to the floor. Measure from the high point of your shoulders down the length of your back.
Find a pair of pants that fit you well and measure from the crotch seam down to the foot opening at the bottom of your leg. Round the number to the nearest 1/2.