Some of the best material I have found on this topic comes out of the book Taming the Money Monster by Ron Blue. The book is a must read for anyone who is constantly finding themselves struggling with debt issues. In fact, at the time of this writing you can buy a used copy on Amazon for a cent, plus shipping and handling.
Ron Blue’s Four Decision Making Rules for Borrowing
Rule #1 – The Common Sense Rule
“the economic return must be greater than the economic cost. To state it another way, when money is borrowed, the thing that was borrowed to purchase should either grow in value or pay an economic return greater than the cost of borrowing”.