first, never use impressions unless its for vocal excise or fun only. Those help you find what your range is but not get you parts. There was a kid who kept trying to spund like johnny yong vosh, if they want JYV they will get him.
Second, learn how you place your mic find an appropriate spot so you can PROJECT your voice with out shoving it your mouth.
3rd, PROJECT YOUR VOICE. People avoid this because of neighbors or family and are embarrassed. Dont be, you are performing and if the part calls for it then say the corniest line with pride.
4th, Dont be afraid to fuck up. This is an art form and a very easy one at that, so messing up once in a while.
5th, Director says jump, ASK How high? Meaning you must listen and do what your director asks. Check your ego at the door, and dont joke around too much, I did and I got in trouble in the past.
Finally, make your own demo tape with roles that you feel fit your voice. put it together in a 1min file. You can make multiple if you like but never go too long. Time is precious and you have to wow them with in 1 minute. Trust me its easier than it sounds.
Thats all I can think off of my head. Acting is extremely easy, you just have to do 1 thing, pretend.