James, for all of its straightforward, practical advice, has its share of conundrums, and verses 9-11 model this well.
9 The brother of humble circumstances should boast in his exaltation, 10 but the one who is rich should boast in his humiliation because he will pass away like a flower of the field. 11 For the sun rises with its scorching heat and dries up the grass; its flower falls off, and its beautiful appearance is destroyed. In the same way, the rich man will wither away while pursuing his activities.
The first trouble with this passage is its relationship with the verses before and after, but I'm going to leave that alone. The other trouble is this: is the rich person James describes a Christian or not? Both interpretations lead to biblical applications, and both interpretations are amply supported by scholars, but I am convinced that James has two Christians in view. I'll spare you the technical reasons, for what is most important is the message James has for these two believers: Whether a Christian is poor or rich, humility is required, and that humility expresses itself in different ways.
If one is poor, says James, he must boast about (that is, set his heart on) his coming wealth. Put another way, he must swallow his pride and humble himself, accepting his earthly circumstances for what they are with hopeful anticipation for the eternal wealth coming his way. So for the poor person, humility expresses itself in the form of boasting about heaven.
For the one who is rich, humility expresses itself in the form of finding one's identity in the humble Jesus, not in the high social standing and other forms of identity the world might give him because of his wealth. As Douglas Moo puts it, "The rich believer is to boast not in his wealth or his elevated social position, but in his identification with Christ and his people, a matter of “humiliation” in the eyes of the world."
So, whether one is poor or rich, he is to view himself not in terms of material well-being, but spiritual well-being. I can think of no better definition of humility: viewing oneself in terms of one's spiritual well-being. And if one is poor, that humility will express itself in the form of boasting about heaven. If one is rich, that humility will express itself in the form of embracing his identity as one among the "humiliated." Whether a Christian is poor or rich, humility is required, and that humility expresses itself in different ways.