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.
When you go all-in with one training philosophy - whether it's bodybuilding, powerlifting, or CrossFit - you set yourself up for a lifetime of repercussions, including lots of embarrassment.