I say your the true size or 1/2 size smaller, or the smaller of the sizes you fit. For example if you fit a 7.5 usually and every so often a 7, get a 7.
I always order 1 full size down. if you go with your normal size, after a few wears your uggs will stretch out then feel really loose. I normally wear 6 or 6.5 but with uggs I always order it in a 5. I own 6 pairs and love them all. Although my older pairs are so loose now gotta to toss them.
Order one full size down.........instead of 7 order 6! They stretch a bit after some time...so you want them to start comfortably snug. This is for the UGG brand