Have realistic expectations-quitting isn't easy, but it's not impossible either. More than 3 million Americans quit every year.
Understand that withdrawal symptoms are temporary. They usually last only 1-2 weeks. Know that most relapses occur in the first week after quitting, when withdrawal symptoms are strongest, and your body is still dependent on nicotine. Be aware that this will be your hardest time and use all your personal resources, willpower, family, friends, etc. etc.
Know that most other relapses occur in the first 3 months after quitting, when situational triggers, such as a particularly stressful event, occur unexpectedly. These are the times when people reach for cigarettes automatically, because they associate smoking with relaxing. This is the kind of situation that's hard to prepare yourself for until it happens, so it's especially important to recognize it if it does happen. Remember that smoking is a habit, but a habit you can break.
Realize that most successful ex-smokers quit for good only after several attempts. You may be one of those who can quit on your first try. But if you're not, don't give up. Try again.