drink water like it's your job. this helps tremendously. never scream, and don't try to talk over people at concerts. don't sing early in the morning. your voice has to wake up just like your body. start off by humming and drinking water, and work into singing.
as far as physical things go, warm up every day. working out your voice is exactly like working out your body. if you don't do it every day, it'll become weak and grow tired, and lose definition. as trite as it sounds, singing scales is the way to go. here's my warm up routine:
start in the lower-middle end of my range, and sing "ma, may, me, moe, moo," and go up half steps until i reach my top. then i go back to the note i started on, and work down till i bottom out.
next i start on the lowest note i can sing and sing the root, third, fifth, hold the octave w/ vibrato, fifth, third root. so for example: c e g c g e c. go up until you have to flip into headvoice, or you are straining to much.
then i just go straight up and down a major scale going up by half steps.
next i do a major scale in threes, meaning: cde,def,efg,fga,gab,abc,bcd, cde. again w/ half steps.
this really wakes the voice up and gets me ready to sing. i started off as a bass II (low as you can go) and w/ time have extended my range to a tenor II without head voice.