Fr. Robert Barron has a fantastic short video on Word on Fire (Youtube) where he talks about the results of our actions, in particular where it concerns sexuality. I can'r find the video now but it appears to be in print form in www.catholiceducation.org/articles/philosophy/ph0056.htm, an article reprinted at Catholiceducation.org. He cites JPII's Love and Responsibility as a reference when he describes the less obvious outcome of every act. Yes, there is usually the obvious consequence, you usually can't miss it. A good example might be an unintended pregnancy, but Fr. Barron discusses the subtle consequences in that video, which might be trivialized or simply get overlooked. That is the gradual undermining of one's character in every act that adds up in the end. Like a bad habit a professional athlete might pick up if not too careful, it pays to note how it can bring that person to ruin in the end.
I often warn my kids: "practice makes perfect." I think this is a good way to explain this gradual erosion of one's moral character. St. Josemaria Escriva notes the Psalmist's imagery of "little foxes" that destroy the vineyard, and that has taken on greater meaning for me when considering, if I'm totally honest with myself, how I backslide and veer off-course gradually if I'm not paying attention.
Things add up: what exactly am I piling up for myself? Practice makes perfect: what am I perfecting? One step at a time: where am I headed anyway? One day at a time; one act at a time: who am I becoming?
Fr. Barron hits the nail on the head when he points our the fallacy of saying to oneself "but deep down inside, I'm a good person." Each act changes who that person is, deep down inside.