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.
Training raw versus with gear, a strong grip, and the importance of hitting PR's even when your life's in the dumper. You got questions, the Hellraiser has answers.