Paul sums it up perfectly when he says in Colossians 4.6:
"Let your speech always be with your grace, seasoned with salt, that you may know how to answer each one"
When we speak, God's loving and gracious nature has to come across. This is less something that we can force to happen, and more something that happens as a result of total immersion in God's word. The bible tells us that out of the abundance of the heart the mouth speaks (Luke 6.45, Matthew 12.34). What this means is that what we say, is a by product of how we actually feel, of what is really in our hearts.
God's word acts as a mirror for us, so that the more time we spend with God, the more we receive the revelation of His love, grace and mercy, and the easier it is for us to impart that same grace to others.
"But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as by the Spirit of the Lord."
- 2 Corinthians 3.18