“The dividing line that keeps student debt in the good category involves the amount you borrow,” Ms. Cagan writes. “While most people ask ‘how much in student loans can I get,’ the right question is ‘how much in student loans can I afford to pay back.’”

Her rule of thumb? “Borrow in total no more than your realistically expected starting salary when you graduate.”

She offers similar advice when it comes to mortgage debt: Don’t take out the biggest one you qualify for, because it is bound to strain your budget.

She offers two other mortgage ideas I like.

First, say you love the idea of paying off your mortgage faster, via a 15-year mortgage, but are concerned about the substantially higher monthly payments — 35 percent to 50 percent more, according to Ms. Cagan — when compared with 30-year loans. Seek out a 20-year mortgage instead. (If you find that you can manage those payments easily, you can always add a bit and pay down the mortgage even faster.)

Second, say your current mortgage rate is relatively high. The average rate offered recently for a 30-year fixed-rate mortgage was 3.7 percent; for the average 15-year fixed, it was 3.15 percent, according to Bankrate.com. Refinancing is a project, and it involves fees. But, she says, you may not have to go through the trouble of refinancing.

The alternative? “Ask your lender to lower your rate,” Ms. Cagan suggests. “For borrowers with a perfect payment history and solid credit, lenders will often say ‘yes’ to this request so you don’t refinance with another company.”