How can I improve my voice/vocal range?

Every year, my school goes to this convention (next on is on October), and this year's will be my last. I join the choir every year, and maybe a quartet or trio. I'm an alto, but I have also performed the soprano part of several songs, although I can't go quite as high as my friend, who is strictly a soprano. I'd like to know if there are any other vocal exercises or such that will help me improve my voice or vocal range. Breathing exercises and vocalizations would help as well. Other techniques to help me sing louder or with more power would be helpful as well, as our choir conductor says my voice is nice, but I need to sing loudly. (I'm generally a shy person).