In the book of James Chapter 3:8(NIV) it says 'but no human being can tame the tongue. It is a restless evil full of deadly poison.' The entire chapter of James 3 deals with problems with the tongue or basically the mouth and what we say.
I have dealt with this many times previously in my writings because indeed it is hard to tame the tongue. We say something and then go "why did I say that? I could kick myself." Tongue control requires much discipline. You may think you are just fine. You do not cuss, lie, gossip, and think you pretty much have it under control. But do you really? What about talking about people innocently such as commenting on someone's appearance maybe saying they are too fat, you don't like their hair, or the clothes they have are ugly? Who are we to judge anyway?
Ephesians 4:29(NIV) is an excellent verse to memorize. It says 'Do not let any Unwholesome talk come out of your mouths but only what is helpful for building others up according to their needs, that it may benefit those who listen.' Talking bad about others does not solve anything and only has you standing in judgment over one of God's creatures. That is something we need not do. If this is your weakness just remember the saying "if you do not have anything good to say then don't say anything at all."
PRAYER: If I am not careful Lord I can find myself being critical of another. I know deep down this is not right. You did not create my mouth to criticize others with but you created it to praise you and encourage others and show love. Help me Lord as I choose good words to build up and help me not murmur about others. In Jesus' name. Amen.