A simple, repeatable way to take the measurements that actually matter.
Measure over the undergarments you will actually wear with the finished garment. This sounds obvious and is the single most common reason a pattern that was drafted correctly still does not fit.
Keep the tape snug but not tight — you should be able to slip one finger under it comfortably. Stand relaxed, breathe normally, and do not hold your stomach in. A measurement taken while braced produces a garment you cannot sit down in.
Get someone else to take your back measurements if you possibly can. Reaching around yourself changes the position of your shoulder blades, and the back width comes out short every time.