A friend of mine attended a workshop where a young woman singer insisted on standing on one leg and clutching the nearest object for support when she sang - one of the worst things you could possibly do to create unwanted tension.
Most habits are caused by nervous tension and this can be controlled and reduced by simple exercises that can be shotened the longer you do them.
When I worked in workshops, the practice we adopted was to ask singers to prepare a number of songs and perform them in front of the group for critical analysis and suggestions of improvement.
Aside from the practical help of having a group of friends assist you to see possible alternatives to what you are doing, once you have undergone an experience like this you will never feel nervous in front of an audience again (or at least, never allow it to spoil your performance) - having returned to singing after decades of not doiing so, take my word for it.
A baptism of fire!
Jim Carroll