Measure your finger at the end of the day when your fingers are at their largest.

Consider the width of the band, a particularly thick band will feel tighter than a thin band so you may want to select a full size up, especially if you are in between ring sizes. 

Do not measure your hands when they are cold as they can shrink up to half a size!

Alcohol and salt can make your fingers swell, so refrain from taking your measurements after cocktails and appetizers.