I believe there is one sin in the world, and it's one that most of us succumb to almost constantly. So it's obviously not killing or stealing or evil speech, acts listed in the Ten Commandments and the Five Precepts that are deemed wrong or unhealthy. See, these acts are bad, but none of them would be committed if it were not for the one sin about which I'm speaking: a lack of mindfulness.
To be present in the here-and-now, to be aware with penetrating insight into that which is occurring now inside you, this is mindfulness. The past is over, and the future is yet to come, so what use is there to dwell on them? Realize that I am not knocking reflection on the past and planning for the future. Mindfulness is when you decide it's time to plan, you plan; when it is time to reflect on past actions, you reflect on past actions; when it is time to reminisce, you reminisce. When you do not wish to engage in those activities, then you do not.
I made the claim that none of those heinous acts (murder, stealing, etc.) would be committed when one is mindful. But can't one mindfully steal something? Actually, aren't the best thieves mindful because they're so aware of what's going on around them, and thus avoid getting caught? No, and here's why. True mindfulness does not refer to being mindful of the external environment (like the thief), although this, too, is present when mindfulness is present. Rather, mindfulness is being fully aware of that which is going on within you. See, when one is mindful, one will not engage in murder because the emotions that fuel murder (anger, jealousy, et al) will be noticed by mindfulness and thus will be analyzed and defused. Same with our master thief—if he were mindful, he would notice that greed is fueling his thievery and would not engage in such activity. Gossip? Rooted in attachment to our ego in most cases, or anger/vindictiveness in others. Hence mindfulness would prevent us from gossiping.
The best teaching on mindfulness that I have seen is Sayadaw U Pandita's book, The State of Mind Called Beautiful. He also has an online teaching entitled In This Very Life. Mindfulness meditation instructions are listed in the first chapter of In This Very Life. They reflect, in lesser detail, what he teaches in the book. There is also a section under Chapter 4 of In This Very Life entitled "Mindfulness" that is a good read.