How can i get more power in my voice:without a teacher?

i plan to take vocal lessons again in the winter however i was wondering if there was anything i could do over the summer that would help me increase my vocal power. I have a musical coming up in november and i want to make sure my voice is strong enough to handle it. My issues are as stated: when i hit high notes they dont come out in full power, when i belt/mix my tone is partially uneven and wavers a bit, and i have a slightly nasal tone when i sing. so i was wondering if there was anything i could do on my own to help correct those issues as well as increase my vocal power. myabe even some songs that i can practice that will help me get there.