How to avoid the tooth from decay under the crown? is there any procedure to avoid the crowned tooth from future decay?

Simple: Good oral hygiene. Flossing and brushing.

Diet & oral hygiene: Diet most important. You can brush and floss all day long--doesn't do a thing if your diet is high in sugar and acid (think soda, lemonade, sweetened coffee/tea) or you have gerd. Great oral hygiene will slow decay rate, not prevent it. Decay is primarily a chemical event slowed by good diet, healthy saliva and by controlling plaque accumulation.

Clean well and have : If it is a well fitting crown, be sure to keep the gumline area clean and have your dentist check the margins of the crown at you regular checkups.

Careful Hygiene: A crown won't decay, but the tooth that is supporting it can decay as easily as any tooth. You still need to brush and floss the tooth as you would any tooth, and avoid high sugar food and drinks habitually. If you do drink or eat high sugary foods, make sure to rinse with water or ideally brush soon after. A prescription toothpaste with high Fluoride content can help protect the margins also.