Shielding:
Mastering shielding techniques will greatly help you in your battles. Each character has a built in shield that enables them to absorb damage. When you hold down the shield button, you will be surrounded with a protective bubble that blocks damage. Remember some attacks and items will hurt you even with your shield up.
Shield Break:
Be careful to only use your shield when necessary. Whether you are taking damage or not, your shield's energy slowly drains with every second it is up, getting smaller and smaller. It shrinks quickly each time it gets hit. Once it gets too small, it will shatter. When your shield breaks, the concussive blow stuns you for a long time. This leaves you wide open to be hurt badly. If Jigglypuff's shield is broken, she will instantly die. Shielding is for those moments when you just have no time to get away.
Shield Reflect:
The only players I have ever known to use this technique consistently are max level computer players. Most people lack the split-second timing to pull this off. I can only occasionally get it to work myself but it's great when it does. If you activate your shield at the exact second a projectile or thrown item hits you, it will not just bounce off or be absorbed, but it will be flung back at your opponent. The surprise of this is almost always enough to ensure that their attack comes back to hit them while they're wide open. Last time I did this, I knocked Samus' fully charged energy ball right back at her and killed her.
Every man dies, but few truly live. Live your life to its fullest, every day as if it were your last.