Time and Temperature
Cold, early mornings tend to shrink your fingers. Measure at the end of the day with warm hands for accuracy.
Thick or Thin?
Thicker bands can feel tighter than thinner styles, so you may want to select half a size to a full size up.
Knuckle Konundrum
If the base of your finger and knuckle vary in size. Measure both and pick a size between the two.