Letchford told me the "biggest plank killer" (aka, the destroyer of form) is an anterior pelvic tilt, and the remedy for it is "working to improve the pelvic range of motion."
"It's hard to get into a posterior pelvic tilt [the correct form] if your hip flexors are too tight. You need to release fronts of your thighs and hip flexors with massage balls, massageing the low back. Create space and awareness of tension that you're holding."
Grab your massage balls (Letchford's preferred tool), or a tennis ball, lacrosse ball, TheraGun, or foam roller. Get creative, but the gist is to have something that can both soften fascia (the tissue that connects muscles) and activate specific muscle groups around the pelvic area.
She had me target the TFL (tensor fascia latae), psoas/hip flexor, tops of the thighs (quads), the piriformis, and the glutes (minimus, medius, maximus — that whole butt and hip area). Think: tops of your legs, where your leg becomes the hip, the hip region, and the booty.
Spend about 60 seconds on each area to bring more awareness to these muscles and create better stability (and watch her hip and low back release video if you need guidance).