Don't overlook the possibility that your investments will mature at an inconvenient time. It's not unusual for a bond to be called earlier than you expect, at a time when interest rates are too low to replace that investment with a similarly profitable one, Mervine says.
The cure: a healthy variety of maturity dates. "You need to think about not having all your investments coming due at the same time," he says. "Every year you'll have something coming due that can be repositioned into the longer-term interest rates."