Teeth can become stained over time due to a number of factors most of them external, smoking, driniking coffee and tea can all lead to stained teeth. Aside from external factors teeth can also become stained due to bacteria build up and poor oral hygiene.

How to prevent stained teeth.
The best method to prevent excessive staining is to practice good oral hygiene, you should brush your teeth after every meal and try to floss once a day, preferably in the evening. Visiting your dentist twice a year for a checkup and cleaning will also help prevent getting stained teeth.

Can I whiten my stained teeth?
So you're a big coffee drinker and smoker, stained teeth will be an inevitable part of your lifestyle but don't worry there are many methods to whiten your teeth.

Teeth whitening.
Whitening toothpaste cannot whiten deeply stained deeth but can help remove surface stains. If you are a big smoker then using a whitening toothpaste may slow down the staining from smoke.
The only way to remove stains is by applying a tooth whitening product, many come in the form of a gel you place into a mould which is then placed on your teeth, and maybe worn ovenight, slowly whitening your teeth. Consult with your dentist first, he can best advise on what product is right for you.
Crowns and veneers are also used for stained teeth but more as a last resort as it's a bigger and more expensive procedure. Crowns and veneers don't whiten teeth but cover the stained teeth. Consult your dentist for the best solution to your stained teeth.