It's been almost three months since I have last smoked a cigarette. (Official as of April 8.) When it comes to finally kicking a habit that you know is bad for you but you continually do it all it deals with is will power. First and foremost though, one must want to actually want the change. This is entirely important, because if you don't then you are only fooling yourself and you'll end up back in your same habits.
Once you've decided you want change, make the change. The first couple of weeks are the roughest, and if you can make it through these then you can do it. But it isn't hard to resort back to your old ways if you aren't careful. One must realize that it takes will power at this stage to not go back. After three months of not smoking I don't find everyday things to bother me and want to smoke, but drinking is the huge issue. This is when I think to myself how long and well I've done so far, and if that doesn't help I think of the reasons why I wanted to stop.
As time caries on these urges will be die down faster and faster until they don't exist anymore. (In some cases though, they may never go away completely but because rather small instead -- as if to be gone.) Overall, just remember that you want the change -- it's up to you if you succeed, as it is your life and you control everything and have no one else to blame but yourself if you fail.