When you tell someone you will call them back, call them back. When you tell someone you will be there by 3:15, be there by 3:15. When you tell someone you will complete a task, complete the task.
Conversely, the most effective way to destroy trust is to say things you don’t really mean, or worse, to say things you know are untrue. It’s a mistake to think that saying “I’ll call you later” and not following through on that commitment (yes, it’s a commitment) doesn’t influence how much people will trust you. It may seem like a small thing to you, but the people around you shouldn’t have to discern between something you said and something you actually meant.
Doing the things you say you will do isn’t difficult, but it does require you to be intentional about what you tell people. If you don’t think you can follow through on something you are about to say, don’t say it. If you only make agreements you can keep, then you set yourself up to be trusted. When you agree to things you can’t actually accomplish, you become your own worst enemy.
No one is perfect and there will certainly be times when circumstances derail you from accomplishing something you agreed to in good faith, but please let these times be the exception and not the rule.