I think humility in this sense is referring to a characterstic or quality of personality, which is either innate, or aquired. Those that are acquired are acquired slowly, over many interactions or situations where the behavior is engaged in and either reinforced, discouraged, or extinguished based on feedback- or lack thereof.
I think, as a parent, the best one can do (when intending to encourage development of certain characteristics) is to:
- Model the behavior that embodies the value or characteristic
- Normalize the value in terms the child can understand
- Reinforce and encourage any behavior(s) the child engages in which conform to the value/characteristic
- (hardest one) Be patient and understanding, especially when this value either seems unimportant to the child, or that they don't understand the value of it
More concisely, I don't think you can or should try to "teach" a child to have humility, or any other value, because that is not how values are ingrained. Rather, you could teach the child about, or the values of humility (and/or any other virtue), and then model and reinforce the behaviors that conform to/embody it.
In my experience, I know that modeling is important to children because I see it very clearly in action. I see reflections of my own good qualities and mistakes in the actions of my children. This is especially true when I notice a change in behavior that reflects my own, or I get an explanation of "because that's what Daddy does!". I also find that they much more often copy behavior that they see than they listen to instructions or commands, or even follow rules.
A concrete example is with honesty. Honesty is a very important quality to me (as I believe it is to most), and I am very clear with my children that lying is never the right thing to do, whether real harm occurs or not. Lying is wrong per se, since it always causes real harm in a trust-based relationship.
With that in mind, I make a very strong conscious effort to never lie, even white lies to, or in front of, my kids. This has the added benefit of making me a better person in general (and living up to my own standards), on top of modeling behavior I expect out my kids. This works exceptionally well with the 3-year-old
As a side note, this doesn't seem to work at all with the 11-year-old, who does not appear capable of admitting a lie to save his life, but that I think is another story entirely. I have, however, noticed that he never says, "But you lie to get out of trouble!"