If you want to look behind but struggle to hold a straight line, try moving a hand to the centre of the bars when taking the other hand off.
Practice at the back of the group before trying it mid-pack.
If you want a perfect demonstration, watch the chairman!
Also, if you need your hands on the brakes make sure you upper body is as relaxed as possible before turning your head. If shoulders are tense or arms locked out straight the bars will turn unintentionally when you look behind.