I find myself constantly going back to the beginning.
I think one of the best ways to feel and get corner development is to do the harmonic slurs every day. Keep the embouchure’s middle relaxed and feel corners tight closer to the aperture. When talking embouchure, it is imperative to recognize there is a shape change that involves more than changing corners. I found that my jaw and tongue respond with the changing embouchure as if it is one system all parts tied together.
One of the struggles that send me back to the beginning is old habits. Corner development does not involve rolling the lips in and out. It seems when I play my best above the staff the middle of lips stay pretty much the same other than the obligatory changes in the corners, tongue, and jaw. When I let old habits meld with some new ways, a train wreck is in the making.