(1) Back (Top Line) Measurement
The first size you need to know is what is referred to as the Back Length, a/k/a Top Line Measurement.  This size is obtained simply by measuring along your dog's backbone- from the base of where their collar rests to where their tail rests.  Every designer is different, so you may also need to take your dog's girth (chest) measurement into account and not rely solely on the top line measurement.

(2) Neck Measurement
The second size you will need to be very familiar with is your dog's Neck Size.  To determine this size, measure around your dog's neck with a tape measure.  Obtain the exact size of their neck and then add just enough room that you think will be necessary for their comfort.

(3) Chest Measurement
The third size you will need to know is your dog's Chest Measurement, sometimes also called the Girth Size.  To accurately determine this size, measure your dog's chest at their widest point, which is typically just behind their front legs.  If your doggie has a larger girth (perhaps from enjoying too many treats!), you may want to choose one size larger in their top line measurement, so the garment will not be too snug around their belly.  Also, this size (chest) usually corresponds to what your dog's harness size will be.