For voicework, I also find that "open your mouth more" and "keep your voice pitched as low as you comfortably can" are often helpful suggestions. Depending on who I'm working with, exercises to open up the chest are helpful too (that is, bring the shoulders down and back, straighten the spine, let the skull "fit" on the end of the spine, etc.). Of course, posture work is useful for actors for other reasons as well.
I have often thought that pranayama work ought to help, also, though I don't know much of anything about it and haven't seen much benefit from what little I do know.
@the other dave, those are excellent for singing and, when actively used, social situations, but there are other techniques which are more passive. The Khargyraa, Tuvan, Diaphragm Breathing, Nasal Passage Opening, and some more general speech techniques including speaking slowly, pausing often, knowing when to gesture, all of these contribute more effectively to your impression than the techniques you mention, which fade as soon as you get caught in the moment.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.