James is speaking of an attitude, not of a mantra. It's not the words that are important, but rather, the perspective. Tracing back to James 4.4, this set of verse is another example of how worldliness can cause people to neglect God and arrange their lives as though He didn't exist, assuming that we are in control of our lives. The sin here is arrogant presumption—feeling secure enough to leave God out of one's calculations.

James isn't rebuking their planning, but only their attitude. If life is so fragile and changeable, so are our plans. We shouldn't build hope on our plans. He is not suggesting that God make the decisions for us, and then let us in on the secret, but that as we make our plans we are continually aware that God’s sovereign will is a contingency to be included in our attitudes.

Am I misinterpreting this passage or do Christians just ignore it?

James 4:13-16 says,
13 Now listen, you who say, “Today or tomorrow we will go to this or that city, spend a year there, carry on business and make money.” 14 Why, you do not even know what will happen tomorrow. What is your life? You are a mist that appears for a little while and then vanishes. 15 Instead, you ought to say, “If it is the Lord’s will, we will live and do this or that.” 16 As it is, you boast in your arrogant schemes.

In everyday life, most Christians use the former phrase rather than the latter. Now I’m not saying that you must always say “If it’s the Lord will...” before every sentence. But shouldn’t Christians at least get in the habit of doing this? Maybe I’m misinterpreting the passage.