I attended the Columbia School of Broadcasting for a short time back in the late 70s, thinking I might want to be a DJ on the radio........ha! Well that didn't pan out, but I did learn how to improve and strengthen the voice with an exercise they taught us.......
From the diaphragm, bray like a donkey "Ahhhhhh", "Ooooohhh" and "Uuuuhh" for long, deep breaths, increasing power and focus as the voice warms. Vary the pitch and concentrate on keeping the tone steady and smooth. Works for me, and easy to do while driving to the gig!