"Whoever guards his mouth and tongue
Keeps his soul from troubles."
"But no man can tame the tongue, it is an unruly evil, full of deadly poison."
The tongue reveals what is in the heart and it cannot be tamed. Guarding the tongue is an active duty, one that must be always on out minds, not something that we can just put on autopilot. The little tongue can work great things for God as well as destroy people, and it is so much easier to destroy than to build up. But as Solomon points out over and over again in Proverbs, the wise person guards their words even though it's hard. To avoid strife, anger, and destruction, we must guard out words.
It's too easy to throw out a criticism or cruel comment without thinking, and I've been guilty of that more times than I could possibly count. I don't have any great words of wisdom or advice for how you can guard your mouth in five easy steps. All I can say is that it takes prayer, God's strength, and a conscientious effort to watch your mouth. We need to stop making excuses for ourselves when we say things we shouldn't or trying to justify our words and admit when we are wrong or have said something we shouldn't have.
The book of Proverbs constantly contrasts the wise man and the foolish man. The foolish man says whatever pops into his head, leading to trouble, but the wise man is careful with his words and only says that which is good and beneficial. We need to be striving, through God's strength, to be like that wise man, because if we try it on our own we will just end up like the foolish man.