Maintain solid spinal alignment. Most people who are new to this will want to cheat the single-arm plank by creating cervical hyper-extension and tilting their heads up. This strains the neck and places excessive tension on the lumbar spine by causing the hips to sag.
Focus on keeping a tall and elongated neck throughout rather than tilting the head up or down.
Sure, the big 3 lifts are great, but experienced lifters need to stop chasing numbers for a while and fix their weak links. Here's why and how to do it.