Although a debit card is technically like cash or a check, in reality it feels much more like a credit card. Because you don't hand over cash, you may feel as though you're not really paying for this purchase, much like when you use a credit card.

And if those funds are earmarked for other needs (like paying off your debt or saving for a vacation), you'll end up without enough money to meet your needs by the end of the month.

If you take $80 in cash to the grocery store, you'll be very careful not to exceed that amount with convenience foods. But if you take a debit card, you're not likely to be nearly as careful. Put the debit card in the same place you put the credit cards — your best bet is in a safe-deposit box.