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 I first approached corrective and performance exercise specialist Paul Chek about writing for Testosterone, his first concern was whether I'd choose to soft-peddle his article. In other words, he was worried that I'd try to "dumb down" his material.
Sticking points are much like those really talkative, naked old men in your gym's locker room: you'd rather avoid them! Nothing's more frustrating than making good progress overall but failing to improve on the "big lifts" (bench press, squat, deadlift, military press, etc.)