Humility is hard to pin down or define, though we generally know it when we see it. It’s certainly not a matter of running ourselves down, denying the gifts and dignity which God has given to us. From Philippians 2, it looks as though it has more to do with how we see others than how we see ourselves.
The difference between our instinctive tendency to see ourselves as worse than others and Paul’s call to see others as better than ourselves may be subtle but it’s significant. Paul’s way ends up increasing the sum total of regard we have for the world and its people. The other way ends up making the world look less than it is.
When Paul draws on the example of Jesus in the hymn he quotes, his point is not that Jesus thought himself worthless but that he chose to see us as worth saving, even at the cost of his life. So we should see others, whoever they are, as worth serving for Jesus’ sake. That has more to do with true humility and discipleship than does any attempt to persuade ourselves that we’re worthless. In any case, that’s not how Jesus chooses to see us, and I’m inclined to trust his perspective more than my own in this.