One nasty surprise for some retirees with a lifetime of great credit: They have no credit score.

If you're not using credit -- your house and car are paid off and you have a few cards you never use -- then there is no activity for creditors to report to the credit bureaus to generate a score. At the same time, if issuers notice you haven't been using their cards for more than a few months, they may decide to close your accounts.

From the issuer's point of view, "it doesn't make sense to have that exposure not generating income," says Ulzheimer.

One fix: "Once every two or three months, put some kind of charge" on your card, says Paperno. "It doesn't have to be much -- coffee, a candy bar."

Just how long a card can stay inactive without the lender closing the account will vary with the card and institution, says Paperno. "Basically, that's up to the lender."

Small, sporadic usage combined with paying bills in full will keep all of your cards oiled and in working order. Since you're not carrying a balance, you're maintaining credit without racking up debt.