In the yotzros of Parashas Parah we say:
מי יתן טהור מטמא לא אחד
This can be translated, "How does one become pure? How does one stop sinning? The answer is לא אחד ,by saying "No!" once and for always. Say "no" once, and you won't need to say no again.
To explain this idea, we share the following: An alcoholic recognized that his drinking was destroying his life, so he decided never to drink again. Around ten in the morning, he became thirsty for a drink, but he controlled himself and didn't take it.
At noontime, he started walking towards the bar, his old habits overtaking him. Midway, he reminded himself of his resolve and stopped himself. He wouldn't take a drink, and that was final. He returned home. He had several other tests that day, but he overcame each challenge. Ten o'clock at night, he told himself, "I had a very good day today. I deserve a drink to celebrate." So he went off to the bar and got drunk.
Tomorrow, if he decides once again to stay clean, he will need to say "no" again. Why? Because his first resolve wasn't strong enough. But the teshuvah we are talking about in the yotzros is לא אחד ,when one says "no," once and for always. This is indeed an approach to teshuvah, but there is also the gradual approach.
The Midrash (Tanchumah, Behaloscha 10) says, "The Jewish people said to Hakadosh Baruch Hu, 'Ribono shel Olam, You know the great power of the yetzer hara… [How can we conquer him?] Hashem replied, 'Send the yetzer hara away, step by step in this world, and I will totally remove the yetzer hara in the future. Today, improve in one thing; tomorrow, improve a bit more until you acquire a second nature." This Midrash refers to the gradual approach to teshuvah.
When one sets out on becoming a better person, he isn't always able to make one final and firm decision to change. Often, one needs to change gradually, step by step, day after day. If he doesn't give up, he will succeed.
The Beis Avraham of Slonim zy"a, compared this to someone who drinks a cup of wine. It seems like nothing happened. He isn't drunk. He isn't even dizzy. Then he drinks a second cup, and then a third until he's intoxicated. Now he recognizes that the first cup also contributed to the end result. Similarly, when one overcomes the yetzer hara once, nothing seems to have changed. He seems to be the same person as before. But it isn't so. Something has changed. If he keeps at it, one step at a time, he will see that he has become a new person.
The Tanya (27) teaches: "Whenever one is victorious over his yetzer hara, even for just a short moment…for example, he desires to eat, but he pushes it off for an hour or even for less than an hour, and during that time he studies Torah… Or when one remains quiet and doesn't say what he strongly desires to say…even if it is merely for a brief moment… it sanctifies him… As Chazal say: 'When a person sanctifies himself a little bit below, Heaven sanctifies him a lot from above."
So, two things happen when one improves step by step: (1) Each time he overcomes the yetzer hara, he gradually becomes a better person. (2) Each small step in the right direction is significant because this small deed grants him siyata dishmaya. Hashem will sanctify him and help him win over the yetzer hara.