Take therefore no thought for the morrow: for the morrow shall take thought for the things of itself. Sufficient unto the day is the evil thereof. (Matthew 6:34)
O.K. Wait now. Is he confused? Surely that “look before you leap” thing doesn’t go with that scripture? And come to think of it, isn’t there another scripture that says something sort of different? The one that goes “For which of you, intending to build a tower, sitteth not down first, and counteth the cost, whether he have sufficient to finish it?” (Luke 14:28).
Well, yes, and no. Yes, Jesus did say that think about the tower. No, I’m not confused.

“Look before you leap” is not in conflict with Matthew 6:34. Matthew 6:34 really doesn’t say don’t think about the future. In fact “Take therefore no thought for the morrow” isn’t a very good translation. William Barclay explains:

It is not ordinary, prudent foresight, such as becomes any individual, that Jesus forbids; it is worry. Jesus is not advocating a shiftless, thriftless, reckless, thoughtless, improvident attitude to life; he is forbidding a careworn, worried fear, which takes all the joy out of life.
That, of course, makes perfect sense when you look at the verses that go before.
So that covers the Matthew verse … but what about the verse from Luke? It’s stating an important principle. It’s about counting the cost of building something. It’s good advice — nothing gets built without a cost, and starting something and not finishing it always looks pretty silly. But there’s a question — what construction is being planned? Well, Jesus has been talking about discipleship. That’s what the text is about — if you want to build a truly Christian character and become a disciple, are you prepared to pay the cost? If you’re looking before you’re leaping, you might ask what the cost is? Good question! It turns out to be pretty high: “So likewise, whosoever he be of you that forsaketh not all that he hath, he cannot be my disciple.” (Luke 14:33) That’s right — the cost of discipleship might be everything you have!

So, putting these apparently random thoughts together, what do we have? Don’t worry, but be prepared! Those are important principles for a Christian. The Christian life is meant to be forward looking. Not worrying about tomorrow, but “Pressing toward the mark for the prize of the high calling of God in Christ Jesus.” (Philippians 3:14)