Acknowledge what is wrong
Acknowledging a problem is the first step towards finding a solution or a cure for it. Sometimes a bad habit is like a black mark on one's face, it is apparent to everybody except oneself.
Assess yourself: How often do you catch yourself using abusive words? Are there particular times when you use bad language more than others (for example if you are with friends, when you are angry with someone or frustrated over something). Has anyone ever pointed out or taken offense at the language you use?
If you are honest with yourself and not in self-denial, you will soon realize if you have a bad language habit or not. If you do, acknowledge that it is a sin for you to abuse the gift of speech given to you by Allah in a manner that is unbecoming for a Muslim. Muslims are seen as representative of their faith, and to use foul language even as a joke or to keep up with the crowd that you hang out with, will lead to the conclusion that Muslims swear -- even if you are just one person.
If you don't use bad language but have friends or family members who do, try to make them aware of their habit in a gentle manner.
Accept responsibility and repent
It may sound strange to repent for the use of a few stray words uttered in jest, but think about what the Quran Says (what means): {Have we not made for him a pair of eyes? A tongue, and a pair of lips? And guided him towards the two paths (of success)}[Quran 90:8-10]
Allah the Exalted has informed us that we are responsible for whatever we say and do, and we will be asked about how we used the faculties that He gave us on the Day of Judgment. We will be rewarded if we use our organs and faculties in the best way and for the sake of Allah. We will be penalized if we abuse them and use them to do wrong actions which are displeasing to Allah.
We have to remember that on the Day of Judgment, our physical organs will stand as witnesses for and against us. Allah Says (what means): {On the Day their tongues, their hands, and their feet will bear witness against them as to their actions.} [Quran 24:24]
If we truly believe in the words of the Quran and in the Day of Judgment, we would not hesitate to repent now -- while there is time -- from all our sins and shortcomings, in order to avoid eternal punishment and humiliation.
Discourage others
Muslims have an enormous responsibility to enjoin good and forbid evil in all circumstances and in all its forms -- especially if the evil is a common failing which has the potential to spread to others. People seen as being active and practicing in the community have a great responsibility to show their disapproval of bad language and not let it pass if it happens in their presence. If Muslims allow foul language in their homes and communities to pass unnoticed, it is almost the equivalent of giving it their tacit approval, so that it slowly spreads and becomes socially acceptable. On the authority of Abu Sa`eed, may Allah be pleased with him, the Prophet said, "Whoever sees something evil should change it with his hand (i.e. physically). If he cannot, then with his tongue (verbally); and if he cannot do even that, then in his heart (by detesting it). That is the weakest degree of faith." [Muslim]
Also on the authority of Ibn Mas`ood, may Allah be pleased with him, the Prophet , said, "There was not a single Prophet among those who were sent before me who did not have apostles and companions and followed his way and obeyed his commands. But afterwards other generations came whose words belied their deeds, and whose deeds were not in accordance with what they commanded others to do. Whoever struggles against them with his hand is a believer. Whoever struggles against them with his tongue is a believer. And whoever struggles against them with his heart is a believer. But when none of these things are done, then not a single mustard's seed weight of faith is present."[Muslim]