From Bankrate.com: http://www.bankrate.com/finance/c...spx#slide=
Another common reason for declined charge is risk of fraudulent usage.
1. Make sure that you call ahead for usually large purchases.
2. Take every measure against possible fraudulent charge of your credit cards (e.g., keep only a few cards, use only 1-2 cards, be alert on potential compromise of card information in restaurants, gas stations, and supermarkets).