Nothing can rock a relationship like borrowing money. But if you have to hit up your near and dear, start by explaining what the money is for, says Jean Chatzky, author of Pay It Down! From Debt to Wealth on $10 a Day (Portfolio, $14, www.barnesandnoble.com).

Then volunteer to put the details in writing: amount being lent, interest rate, payback dates. (Find free sample promissory notes at www.formsguru.com.)

“The temptation, especially with family, is to say, ‘Pay me back whenever,’” says Chatzky. “But that rarely works.”

Instead, ensure that your relationship isn’t headed for trouble by “making it clear up front whether it’s a loan or a gift,” says Peggy Post, author of Emily Post’s Etiquette, 17th Edition (HarperCollins, $48, www.amazon.com).

Then give them time — and an out — by saying something like “Don’t feel compelled to answer me on the spot.” Cautions Chatzky, “Don’t make assumptions about how wealthy the person you’re asking the loan of actually is. It’s like looking at someone else’s marriage when you’re on the outside.” So avoid saying, “I know this is nothing for you.” Says Chatzky, “People may have financial obligations you don’t know about.”

And, finally, whether the answer is yes or no, be sure to say thank you.