How Do I Prevent Staining my Teeth or Veneers?

Unlike composite veneers (many can shift in color over time and are somewhat porous), porcelain veneers don't change color over time. Some dentists may use a cement that can yellow over time, but most experienced cosmetic dentists don't.
What needs to be considered are the margins. The EDGES of the veneers might become discolored, so a skilled cosmetic dentist makes sure that the margins are placed in an area that won't be noticed if they DO discolor.
Colored beverages can discolor tooth structure or stain margins. Coffee, colas, tea, red wine, etc should be avoided or used infrequently. Smoking is terrible too.
Routine visits to the dental office can help keep them white as well.

If you have porcelain veneers I would not be concerned about them staining, but your natural teeth do collect stain.  Here are several things to avoid to keep staining to a minimum:

1.  Avoid smoking

2.  Avoid "whitening" toothpastes due to their abrasive characteristics.

3.  Stay away from coffee, tea, red wine, etc.  As most of these liquids are part of many individual's daily intake I would at least consider drinking them through a straw. 

4.  Practice great oral hygiene by using a Sonicare toothbrush and  Waterpik for hard to reach areas. 

5. See your hygienist 3-4 times per year for professional cleanings. 

Follow these suggestions and you should minimize the amount of stain that you accumulate over time.

Stay away from the major stainers- coffee, tea, red wine, cigarettes. Veneers shouldn't stain, but natural teeth and bonding do. 

First, the good news.  Today with the ultra polishable extremely dense porcelain veneers you will rarely get stain on them if you exhibit good oral hygiene.  They are very color stable, and any surface stain that you may pick up can be easily polished off by your dentist or dental hygienist using very fine low abrasive polishing paste.  I've been doing porcelain veneers since they were introduced to dentistry around 1985 and in that time I've only had one patient come in with any significant staining of his porcelain veneers.....he was a cigar smoker.  We polished off the cigar staining in a couple minutes, shared with him how he could better brush and floss after smoking his cigars, and since then he only comes in for his visits with our dental hygienist with minimal cigar stains.

However, natural teeth are another thing.  They can pick up staining very easily!  Worst offenders are smoking, coffee, tea, curry sauce, soy sauce, colas, berries, grape juice, wine, etc.  The tooth enamel can stain both externally and internally, because those offenders just mentioned can stain the outer surface of the teeth along with permeating into the inner surface.  Your dental hygienist can easily remove the external stain, but to get any internal staining out of your natural teeth you should consider teeth bleaching (also called teeth whitening).

There are three more culprits that can cause your natural teeth to discolor.  One is just pain 'ol natural aging.  The second is the nerve of the tooth dying and needing a root canal to stop the infection.  This does not happen frequently, but does happen.  A third reason is something called "traumatic discoloration", which could happen from a traumatic blow to a tooth or the tooth having had braces. 

Avoid coffee, tea, red wine and smoking. These are the most common staining agents. Brush with a soft tooth brush or an electric toothbrush. Avoid abrasive toothpastes or whitening toothpastes.

Floss your teeth daily.

Have your teeth cleaned every 3-6 months at your dentist's office