Building wealth has gotten harder for most people in recent years. But the habits that can make you rich haven’t changed.

It boils down to this: putting aside money, regularly and consistently, that can be invested for your future. You have to leave that money alone to grow, which means you also need an emergency fund. And you must be careful with debt, because the wrong kinds can erode your wealth rather than build it.

It’s a simple formula but one that’s become increasingly hard to implement as incomes stagnate and prices rise. A shocking number of American households — nearly half, by the Federal Reserve’s last count — don’t have enough savings to cover an unexpected $400 expense. Our inability to save has contributed to a 21 percent decline in household median net worth between 1998, the year median incomes peaked in America, and 2013, the last year for which Fed stats are available.

Hardest-hit are households in the lower middle class, which in 2013 meant incomes from $23,300 to $40,499. Their net worth fell by half.

In my latest for the Associated Press, how to use the habits of wealthy people to build for your future.