By cleaning out my closet, obviously!
I have been reluctant to get rid of my 18s and 20s, but today I resolved to do it. I went in with a bag for Goodwill and an eye to get rid of anything that doesn't fit anymore. When I was done, I had a pile of Goodwill, a closet full of clothes that fit, and another pile of "nearly there" clothes that are my 14s and down.
It feels like a weight has been lifted from me, even if it's not from my body itself! Hopefully someone can benefit from my old clothes, while I continue to work my way down through the sizes.