When you make a vow to God, do not delay in fulfilling it. He has no pleasure in fools; fulfill your vow. It is better not to vow than to make a vow and not fulfill it. (Ecclesiastes 5:4-5)
This verse pierces me deep down to the soul. It forces me to think about all those times I brought empty promises before the Lord: "Dear Lord, if you could please help me through X, I'll do Y in return." Time and time again, the Lord helps me through X, but I never even attempt to fulfill Y. Why on earth do I do that?? Do I not take my promises to the Lord seriously? Do I consider Him so loving and forgiving that I find it acceptable to break my vows to Him?
The truth is, I would do almost anything to avoid breaking a vow to a family member or friend, but when it comes to the Lord I am considerably lax. This is something I must make every effort to change - not for the Lord's sake but for my own. The Lord doesn't need anything from me, but I need all He has to give: His fellowship, His wisdom, His strength, His mercy, His forgiveness...
Perhaps someday I'll make a vow to never make an empty promise to the Lord again. Hopefully I'm never that foolish, unless I mean it with every fabric of my being.