You know what they say,...：俗话说，有人说过，人们常说。
相比以前学过的there's a saying that goes，what they say是比较容易想起的表达，引用某人的话就把they替换掉就行了。

【例句】
You know what they say: a friend to all is a friend to none.
你知道有句俗话是这么说的：交游满全国贴心无一人。

Lisa: 'Well, you know what they say – a change is as good as a rest.'
丽莎：“嗯，常言道：调换工作益如休息。”