I'm a firm believer in the value of wearing the right bra size. It can change your life. After all the breast-changing rigmarole of pregnancy and breastfeeding, I went and got professionally fitted for a new bra, and it's amazing how good it feels to wear the right bra size (and yes, I was commiting the most common error of wearing a bra too big around with too small a cup size).
I found this Daily Candy How to Find Your Bra Size video quite helpful for DIY bra sizing.
Of course, Cafe Kim's sister Kristin's tips are invaluable in the earlier post Are You Wearing the Wrong Bra Size?; however, a nice video visual of bra sizing and fit can't hurt.
Go on over and check out the Daily Candy video, led by Linda the Bra Lady.
Another fun bra story: From Brandi to Britney, 11 Bras That Made Titillating History