When rich, think of poverty, but do not think of riches when you are poor.

解释 ( paraphrase )
富裕的时候，要常常想到可能有贫穷的那一天；不要等到贫穷了，再来回忆富裕时的生活。
这条言语告诉我们：勤俭节约是管理家庭、治理国家的基本方针。富日子要当穷日子过。不要把钱花光了才后悔。

This proverb indicates that frugality is the best policy: Be frugal even when you are rich, and do not dream of riches when you are poor, but work hard and be thrify.

将：把。
      The same as 把.

待：等到。
      To wait.

造句 ( example )
“常将有日思无日,莫待无时想有时”我们要珍惜现在的富裕生活。