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.
Although I'm better known as someone who works with high-performing athletes, I started out–like most coaches and trainers-by working with "everyday folks", and interestingly enough, after 20+ years in the game, something unusual has happened: