In summary, your capped teeth are less prone to decay than uncapped teeth, but they are still at risk; take care of them.
There are three factors that must occur at the same time for you to develop cavities. First, cavity-causing bacteria need to present in your mouth. Next, you need an exposed tooth surface for the bacteria to work on. Finally, you need available food material for the bacteria to grow on.
The bacteria that cause cavities produce acid. The acid eats away at the outer surface of the tooth, resulting in decay.
Having capped or crowned teeth reduces the exposed tooth area so there is less tooth surface available to attack. However, this is not the primary purpose of crowns placed on teeth. Crowns usually are placed to preserve teeth that have been damaged or decayed. If you are getting crowns placed on your teeth as a result of decay or some other dental condition, you need to take special care to prevent cavities from developing around the neck of the tooth and possibly on the root surface of the tooth. These are areas of the tooth that the crown does not cover, and therefore are susceptible to future dental decay.
You can prevent cavities from forming by reducing the amount of plaque (food particle build-up) and bacteria in your mouth. The best way to do this is by brushing and flossing regularly throughout the day and getting professional dental cleanings as recommended by your dentist or dental hygienist.
Also, teeth can be strengthened by fluoride. A dentist can evaluate your risk of caries and then suggest appropriate fluoride treatments. Using an over-the-counter fluoride mouth rinse after brushing and flossing can also help to protect the tooth. You can reduce plaque buildup by avoiding sugary or starchy foods and by eating less frequently during the day. Each time you snack, the bacteria around your teeth produce acid, so snaking throughout the day is more likely to lead to cavities.
The best advice I have is to avoid snacks. If you can't, then should brush your teeth more often. Chewing gum that contains xylitol helps to counteract the acidity that occurs after eating.