Justification is something done for us;
sanctification is something done in us.
Justification is a once-for-all declaration by God;
sanctification is an ongoing, inward work of God.
Our justification can never be improved upon, because we are seen perfect in Christ;
our sanctification is constantly being improved upon, because we are never actually perfect in this life.
Justification alone gives us the right to enter heaven;
sanctification makes us fit to enter heaven.