"“Salt is good; but if the salt has lost its flavor, how shall it be seasoned?" (Luke 14:34, NKJV)
In this passage Jesus calls His disciples to consider their effectiveness. The analogy of salt is straightforward. If salt loses its saltiness then it cannot used for its intended purpose. Likewise if a Christian loses their Christ likeness then they have no more effective value.
Becoming increasingly effective as a Christian begins first with becoming a Christian. That is a supernatural experience by the grace of God (Ephesians 2:8-9). From our perspective Christ becomes our highest treasure and thus we call on His name. "that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved. For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation." (Romans 10:9-10, NKJV). Just as salt must have the right chemical properties of salt to be effective so a person must genuinely be a child of Christ to be an effective Christian.
Around the Dead Sea region in Palestine the salt there is often contaminated and thus can lose its effectiveness. Likewise a genuine Christian may lose their effectiveness because of impurities or sin. Refining our character is not a matter of will power it is Word power. Jesus said "Sanctify them by Your truth. Your word is truth." (John 17:17, NKJV).
The secret to becoming more like Jesus is by beholding His glory.
"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, NKJV)