Before you measure anything
Decide: Bra or no bra? Your bust level and circumference measurement will change with and without a bra on, so if you plan on wearing a bra with your finished garment, make sure you wear one during the measuring process. Otherwise, make sure you are wearing form-fitting clothing (like a tank-top and leggings) and your typical day-to-day undergarments.
Get help. Stand in front of a mirror and have someone on standby to assist you (especially for those back measurements.)
Use the right tape. More specifically, use a flexible measuring tape (rather than a fabric one, as those tend to stretch out over time.)
Check your posture. Stand upright in a relaxed position with your feet together. When measuring, breathe normally and make sure the tape is comfortably fitted to the body.