As of Aug. 15, 2010, banks must get a customer's explicit consent, often called an "opt in," to charge overdraft fees on single debit card purchases or withdrawals from ATMs.

This explicit consent isn't required for checks, pre-authorized electronic payments or recurring debit card charges. In those cases, the bank can decide whether to cover the transaction, and if it chooses to do so, charge a fee for that service, according to Carol Kaplan, a spokeswoman for the American Bankers Association in Washington, D.C.

Yet, despite this bifurcated system in which consent is required for some overdrafts but not others, one thing is still certain. The easiest way to avoid overdraft charges is to not opt in to the service.

"Some people just shouldn't opt in (because) they are challenged in limiting their spending or spending within their means," says Todd Sandler, head of product strategy at ING Direct, an online bank in Wilmington, Del., that doesn't offer fee-based overdraft protection.

If you don't opt in to overdraft protection and your transaction is declined, you shouldn't be charged a fee.

"There is no cost in a debit or ATM transaction that is declined," Fox says.