Neck: Wrap the tape measure about an inch below your Adam's apple, tight enough for a decent fit, but loose enough so you can still breath. Round up to the next half-inch.
Shoulders: Measure from tip of one shoulder to the other, across the top of your spine and at the base of the neck. The tape should follow your back's natural curve.
Chest: Stand normally and comfortably (without flexing) and have your partner measure at the broadest part of your chest, inline with the nipples, leaving enough room to breathe.
Sleeve: Measure from the shoulder bone to the wrist bone. Simple.
Waist: While avoiding the temptation to suck it in, wrap the tape around your waist at the navel. Slide a finger between the tape and your body to allow some room. Then, buy a belt.
Inseam: Figure out where you want your pants to end, then lose your shoes and find a significant other (or true friend) to measure from your crotch to that specified point.
Skirt Length: Measure from waist line to your ankle.