In information on nicotine & addiction I’ve tried to explain why mind and body become dependent on nicotine, causing problems with stopping smoking.
Your mind is the biggest obstacle you will have when you stop smoking, and I know from personal experience how big an obstacle it can be. But with good preparation and a strong resolve, it is possible to stop.
Craving: Smoking is perceived as a pleasure. For a smoker, there’s nothing quite like the “hit” from a smoke, and nothing to replace it. Unfortunately every cigarette smoked does damage.
Simple things like taking a cigarette out of the box, or rolling one up, the smell of the tobacco, lighting it, the smell of the smoke, inhaling the smoke, the taste and feel of the smoke, exhaling the smoke, the feel of the cigarette between your fingers, flicking the ash, are all things that become hard-wired in your mind and strongly inter-woven in your normal day-to-day life.
Because your mind perceives smoking as a pleasure, it builds strong associations with situations were this pleasure happens;- where you like to smoke; e.g. having a cup of coffee, watching TV, listening to music, having a drink with friends, reading, talking on the phone etc.
When you decide to stop smoking, you must take the time to prepare yourself. Mental Preparation is Key. Try to analyse yourself, 
your behaviour, as best you can, when do you like to smoke ? sometimes you may not even be conscious of reaching for one. This process of self analysis can help you to minimise the number of times you are ambushed by the Dreaded Cravings. For example if you associate a cup of coffee with a cigarette, as mentioned earlier, every time you smell coffee you’ll want to smell smoke. Be prepared;- “I’m going to have a cup of coffee, so I’m going to want a cigarette”, you’ll be more likely to resist the temptation if it doesn’t sneak up on you. Research has shown that Nicotine craving will only last for Three Minutes, in this time it will build to a crescendo and then disappear, so try to hold out.
In addition to craving, it’s possible that one or more of the following may occur:-
Coughing : a cough may develop, or an existing one 
may worsen. This is actually good news, because it means the tiny hairs (cilia) in the lungs, which are often suppressed by smoking, are starting to work again, and are simply doing their intended job of clearing the smoking residues from the lungs. Coughing will pass with time, ask your pharmacist if you need help with symptom relief.
Hunger & Weight Gain : Nicotine is an appetite suppressant in most people, so when 
the levels drop;- your appetite for food may increase.
Also because cigarette smoke is no-longer in contact with inside surfaces of your mouth & nose, your sense of taste & smell will improve, and you will begin to enjoy the sensation of eating with renewed enthusiasm, tasting and smelling things you couldn’t decipher before, when you smoked.
It is often the case that ex-smokers fill the void left in their lives by snacking. If you must snack try to eat low calorie, low fat foods or sugar free gum or mints. The average weight gain when you stop smoking is between 4-8 lbs. However this can be lost after you’ve kicked the habit.
Change in Gut function : in information on nicotine & addiction I explain that nicotine has a direct action on the smooth muscle in the gut. When levels of Nicotine change it can result in a change in gut function resulting in constipation or diarrhoea. When your system learns to live without nicotine from cigarettes, gut function will normalise. In the interim if you need help, ask your pharmacist for advice.
Mood swings, Lack of concentration and Irritability: Because nicotine is a drug 
which affects mental & physical processes in your body, when it’s removed from your system you can expect to experience withdrawal symptoms, these manifest as;- mood swings, lack of concentration, and irritability. The severity of withdrawal symptoms you experience will depend on the size of your smoking habit. Again mental preparation is key, expect the withdrawal symptoms, and tell everybody you’ll come in contact with;- you’re stopping smoking, and as a consequence you may become a bit more irritable and grumpy than usual. Its not easy, but sharing your problems with others will help your situation, and it’s much easier than going it alone. The symptoms WILL pass in a few weeks time so in the meantime do the best you can;- talk about the problems you experience, with family and friends, also seek support from your pharmacy.
Nicotine replacement therapy aims firstly to replace nicotine and then gradually remove it in a controlled way. Using patches, gum, lozenges, sprays, inhalers etc will lessen the withdrawal symptoms, and has been shown to improve your chances of quitting by 50%.