Good posture is going to have a fairly signifigant effect. Sit up straight (or stand) pull your shoulders back and keep a straight imaginary line between the middle of the top of your head and your tail bone when you take the breath in, then fold up in a ball as you run out of air to squeeze out everything, but still keep your airway completely open.
I'm a musician, I play the trombone and low brass instruments as well as several years of trying to sing. One of the huge things both a voice trainer and an instrumental teacher will push from day one is good posture. There are chairs specifically designed to force you sit correctly, they're awful to sit in while you aren't playing but they do work