What really helped me, is to remember to breathe in through the nose, not the mouth. If you breathe through the mouth, your tongue and throat get dry and it makes your voice crack.
Also, use good posture, straight but RELAXED (don't flex or tighten your shoulder muscles), and keep your head up but with your chin slightly tucked down (toward the chest). There are always those dramatic stories that say "she THREW her head back and burst into song...." Well, don't.
Keep your chin down! Tilting backward overstretches the throat (taking the vocal cords along with it) and also increases the risk that you'll get spit down your windpipe.
Diaphragmatic breathing is the best way to always breathe because it brings oxygen deep down into your lungs and allows you to fill the lungs from the bottom, up. Your wind lasts longer and you can sustain a note for longer because of it. When you breathe, in your mind picture filling a glass with beer from a bottle... you don't want the head of the beer to foam up too much and spill over the top, so you kinda let the beer slide down the side of the glass and fill the bottom and fill its way up. When you breathe like that, you should feel your belly expand instead of your chest.
I train in martial arts, and that's how we breathe for that too. It helps oxygenate our muscles and bodies longer so we don't poop out as quickly when we're doing our martial arts routines and sparring and all that.