I meant that it's your friend when you are saving money. Go ahead and play with different numbers in one of those 401(k) calculators.[1] Basically, you deposit the interest from previous years' deposits with your current year. In just a few years, you're depositing quite a bit.

It's an enemy when you borrow money, say with a credit card. Just the opposite of saving, you borrow the previous interest every month. You're paying interest on interest. If you have trouble disciplining yourself, get and use a charge card (such as most traditional American Express cards) that forces you to pay off the balance each month.

[1] This is the first Google hit, no affiliation or endorsement: http://www.money-zine.com/Calculators/Retirement-Calculators...

This means, when you save money, save it as early as possible. Even days matter - plenty of banks calculate interest daily, and add it to the account monthly.

When you borrow money, borrow it as late as possible, and pay it off as quickly as possible. Every day you delay, you also pay the interest on the interest for that day. (and so on and so forth, actually!)

Above all, be prepared to do the math.