Designed to halt piggybacking, FICO 08 ignored authorized-user accounts in scoring. Not so, anymore. Transferring your card balance to save on interest can carry a fee. But you can haggle your way to savings. Using credit cards wisely -- whether it's one or 15 -- is the key to improving the credit score, says a FICO expert. A FICO credit score expert explains how to build good credit -- or patch it up after going through foreclosure. See rates from our national survey of credit cards, mortgages, home equity products, auto loans and CDs.