Commitments should not become hollow promises. It doesn't take many unmet commitments to result in broken relationships. To casually state “I'll take care of that”, but then not do it, creates negative feelings for the slighted party. Everything that happens next gets filtered through a very dark lens; the worse and more frequent the broken promises, the darker the picture becomes.
Considering what's at risk here, commitments should be made thoughtfully and kept religiously. Before offering or agreeing to do something, pause and make sure that what you're agreeing to do is reasonable and realistic to accomplish in the timeframe that's being promised. Our human tendency in the face of challenges or problems is to make them go away, as quickly as possible. An effort to seem highly responsive can result in making a hasty commitment that, if missed, will have exactly the opposite effect to what is desired.
Once you've made a commitment, schedule it and then defend it. Do your utmost to deliver on or before it's expected. In the event you find that you must change the commitment, or cannot accomplish it as promised, it's crucial to communicate openly and honestly with the other party before the deadline comes and goes. There is a nuance here related to changes in “how” the commitment is kept; if you can't do the work in the way that was agreed, you owe the other party a conversation before continuing. A “messed up” result can be just as bad as a “missed” one.
Once a deadline has passed without delivering as promised, however, no amount of scrambling and recovery effort can restore the relationship to the level that existed before. Clients may forgive, but they never forget.
If you want to build lasting relationships, DWYSYGTD! It's a proven way for both you and your clients to enjoy the journey.