STAND TALL
Posture is important:
Stand up with your feet apart at shoulder width.
Make sure that your shoulders are in line with your hips and your hips are in line with your feet.
Keep your chest comfortably lifted and make sure that you're not leaning backwards or forwards.
Allow your knees to bent slightly.
If you’re doing it right, you should feel like you’d be ready to jump on a train if you needed to.
BREATHE WITH YOUR DIAPHRAGM
The diaphragm is that thin muscle separating your thorax from your abdomen. Rather than taking a breath from the shoulders or chest, (which will make you tense), inhale from the diaphragm. You should feel your tummy expanding.
RELAX
Relax your shoulders, your jaw, your palate, your cheeks, your throat and SMILE!
KEEP YOUR CHIN STRAIGHT AS YOU GO HIGHER
This is especially important when you're singing high notes since most singers will lift their chin as they go up in pitch. An easy way to make sure that your jaw isn't pointing up is by thinking "down" as you go up in pitch. Imagine the sound coming out from the very top of your head. Raising your eyebrows might help too.
OPEN YOUR MOUTH
It may sound obvious, but most people sing with a half-closed mouth. Look at a video of a professional opera singer. Can you put 2 or three of your fingers in your mouth, just like the opera singer does?
SING WITH YOUR HEART
Think of the words you are singing and fill them up with your own emotions. Before singing, read the lyrics and understand what the author is trying to convey to your audience.