Say what you mean

One of the hardest things in life to do is stick to your word. It seems like it would be easy enough, just simply don't make commitments that you can't keep, and always do what you say you will do. But we all know life is just not that simple. The important thing to remember is to make the best effort you possibly can and be honest. This is so important in relationships with other adults as well as our kids. We should not say things to our kids that we don't mean or aren't prepared to follow through with. If you need to leave the park in 10 minutes, tell your kids "we are leaving in 10 minutes" and when the ten minutes are up, its time to go! Don't allow them to badger, whine, or beg you into an additional 10 minutes. Also when the ten minutes are up make sure as the parent you are ready to leave at that time as well. So many times I've heard a parent say "we need to leave in 10 minutes" and 15 to 20 minutes later they are still chatting with a friend.
Following through is very important. If you tell a friend you are coming by or that you will meet them somewhere at a certain time do your best to follow through on that commitment. It shows that you have courtesy for others people's time by at least attempting to be prompt.
When life does get in the way and you are unable to follow though on something, make sure you be honest and let the other party know of the circumstances that have caused you to not be able to make it.  In the famous words of Tony Montoya in the movie "Scarface" "All I have in this world are my balls and my word, and I don't break them for nobody." Literally he is saying that he sticks to his word no matter what the circumstances, a very powerful quote.