The idea behind the spiritual disciplines, I gather, is to practice certain behaviors that develop one's personality in a positive direction. It's similar to an athelete's physical training: not only does he practice the specific activity he is planning to undertake (hitting the baseball with a bat, throwing a shotput, etc.) but he has to exercise to keep himself in good shape and to develop the muscles that will help him accomplish the specific activity more successfully. And even though the bench presses, squats, and shrugs may not seem to have any direct relevance to that activity, they do in fact help him to do it better. It's not a matter of trying so much as it is training. So it may not appear obvious how some spiritual disciplines will develop your character in a positive way (like fasting) but other people older, wiser, and further along in their training than you recommend it highly, just as the coach may tell the long-distance runner that he has to do a lot of crunches. We can't expect to do the right thing on a particular occasion if we haven't trained and built up the kind of character capable of doing it, anymore than we can expect to win a weightlifting competition if we've never built up the right kind of body capable of doing it.
Now the idea behind this idea is that we can, through little things, become capable of doing great things or horrific things. If we want to become capable of committing a terrible crime, we can perform little steps which incrementally make it easier. Of course, most people who commit crimes have not intentionally engaged in such a program. But you can see how it would work. Through our daily choices we are making ourselves more capable of great things or terrible things. The further down one road we go, the less able we are to do the things down the other road. If we choose to be angry or bitter or sad, we will eventually reach the point where we can no longer choose not to be angry or bitter or sad. Slowly, as we live our lives, we are taking away our own freedoms. This doesn't mean that the end result involves the removal of our free will, just that if we choose to do the things that develop good character, we will eventually be unwilling and unable to commit terrible acts. If we do not develop good character, our sphere of freedom could still include terrible acts. Of course it's different for every person: some people have an innate disposition for goodness or badness; some people are genuinely satisfied at one level while others will want to travel further down the path; some people will have a wider sphere of freedom so that they can encompass more of the opposite path than they have chosen. And of course, most people do not really choose their path, they just drift through life without going very far down either one -- or perhaps being unaware that they are going down one. Nevertheless, as a general truth we are, by our daily choices, becoming the people we will forever be. We are choosing to have a character that is good or bad or just passive. If we take the idea of eternal life seriously, we will want to have a good character; we will want to be capable of great things, not evil things. Eventually the sun will rise on our characters and turn them to stone, so we'd better make sure they're in a position you wouldn't mind being in forever.
We all want to indulge ourselves. When an opportunity presents itself, we will find excuses why it's appropriate to give in to this particular occasion. The problem with this is that if you don't practice not giving in to such temptations, you'll eventually be unable to refrain from doing so. When I was in the Marines I had a friend who slept around all the time, even though, at any given moment, he had a serious girlfriend. I told him one time that I felt sorry for whatever woman he would eventually marry because she would have a cheating husband. He was (understandably) offended by this. He insisted that he would be completely faithful to his wife. The reason this seemed so implausible to me is that he couldn't even be faithful to a girlfriend. He didn't practice being faithful, he didn't build up within himself the strength to turn women down when they offered themselves to him, so how could he seriously start doing it successfully for the first time once he was married?
This is the idea behind Edgar Allan Poe's wonderful short story "The Imp of the Perverse". Click on the title and read it first, because there are spoilers below, and it's not too long.
Done? OK, so the narrator enjoyed the feeling of rebellion we get when we are told we should or shouldn't do something. We instead want to assert ourselves and do the opposite of what we should or shouldn't do -- this is the imp of the perverse. The narrator enjoyed this so much he committed himself to never denying himself the pleasure of doing something that he felt he shouldn't and vice-versa. Not that he would do it unthinkingly; he would take his time, making sure he wouldn't get caught. But he would do it. So when he realized he "shouldn't" kill a relative for the inheritance, he went ahead and did it, and did it cleverly enough that he got away with it.
Then, years later, he feels very satisfied with himself, realizing that no one will ever be the wiser about his crime. Unless, of course, he were to confess it. But that's crazy, he shouldn't do that.
He had indulged himself for so long in doing whatever he shouldn't do, that he was unable to withstand this occasion. He didn't have the muscles built up to do what he should and not do what he shouldn't. He had taken away his own freedom, his own ability to disobey the imp of the perverse.
A more profane example comes from South Park. In the episode "Le Petit Tourette" Cartman discovers Tourette's Syndrome and pretends to have it so that he doesn't have to filter what he says. This is funny, partially because Cartman isn't really starting from a position of strength on this issue. Anyway, he swears, yells racial epithets, etc., and not only does he not suffer any negative consequences from it, but receives compassion and attention from everyone. Eventually, he manages to get himself booked on a national television show to talk about Tourette's. Of course, for him the only reason to do it is to have a national audience forced to hear whatever he wants to say. But before he goes on TV, he starts saying things he doesn't want to say. Personal things, embarassing things. He has spent so much time just yelling whatever he felt like yelling, that he was no longer able to filter it. Anything that popped into his head popped out of his mouth. Chaos and alleged hilarity ensue. You can watch the whole episode here.
The point is the same. Indulgence is very tempting, but the more we give in to it, the less able we are to refrain from it on occasions where we should, where we want to. This fits into the general perspective of the spiritual disciplines. We should exercise those faculties so that we are not compelled to do the wrong thing because we simply aren't strong enough to resist it. But again, the disciplines aren't about trying to do the right thing when the occasion presents itself, but of training yourself so it's not difficult to do the right thing on those occasions.