Come now, you who say, "Today or tomorrow, we shall go to such and such a city, and spend a year there and engage in business and make a profit."  Yet you do not know what your life will be like tomorrow.  You are just a vapor that appears for a little while and then vanishes away.  Instead you ought to say, "If the Lord wills, we shall live and also do this or that."  But as it is, you boast in your arrogance; all such boasting is evil.  Therefore, to one who knows the right thing to do, and does not do it, to him it is sin.

It appears that James is warning us about claiming "tomorrow" as a given, when tomorrow is never promised to us.  At first glance, James seems to want us to stop looking to the future and focus on the now.  This is good advice.  But there is something deeper in verses 13 - 17.

Focus for a time on the word in verse 15: "will".  James is not telling us to avoid planning, he is telling us to stop avoiding God's will.  We are to look at God's desire for us BEFORE we look at anything else.  Set your priorities according to His will.  Set your calendar, after you search for God's will. 

When we plan our lives apart from God's will, we are "boasting in our arrogance."  Which makes sense.  To think that we can do anything apposed to the will of the Lord is to think that our will is equal with God's will.  Arrogance.

Instead we should say, "God's will comes first in my life - everything else is gravy."