Everyone who has been involved with singing will recognise the importance of the mental state. It is best to be relaxed, whilst energetic and confident.
If you are confident about your voice and singing, it is obvious you will sing better than if you are nervous and doubting. Yet as with any physical activity there are muscles involved which can also be trained. When both the physical and mental are combined one enhances the other - if you feel confident you sing better, when you sing well you feel confident. 
During the 1990s Blue Marsden pioneered an approach to learning singing that incorporated the mind and body. Although he no longer works with singers on a regular basis thousands of people have been helped to learn to sing or improve their vocal confidence and ability through his VOCAL TRAINING WITH HYPNOTHERAPY CD currently unavailable but with plans to reintroduce Autumn 2014.