Don't Smoke- Not only will you improve your health by not smoking, you won't stain your teeth with yellow and brown marks.
Drink less coffee- Now we're not saying after you've had your teeth whitened you have to give up coffee, but do reduce your intake as it gradually discolours teeth. The more milk you use the more diluted the coffee which helps to reduce staining.
Choose White Wine over Red- Red wine dramatically discolours teeth. Eliminate this factor by choosing white wine over red.
Beware of exotic foods- Beware of ingredients in foods which can cause staining of the teeth like Indian and Chinese.
Brush regularly- Brushing atleast twice a day once at night and once at any other time of the day. Use interdental aids- Use interdental floss and inter dental brushes to clean in between the teeth.