Elizabeth Null gave a good tip, if you are sitting when singing. Stretch out on the chair, almost as if you are taking a nap. This opens up the diaphragm and allows the voice to come through.

I sing mostly in song circles, where it almost seems pretentious to stand to sing and so mostly sing sitting down. When performing, with Kendall playing guitar, I tend to stand to the side and slightly behind his chair. There definitely is better breath control when standing IMHO.