Chi dorme non piglia pesci. Those who sleep don't catch any fish. Easier said than done. Il tempo viene per chi sa aspettare. All things come to those who wait.
Ditches are full of hindsight.
What's done is done. Uomo avvisato, mezzo salvato. Forewarned is forearmed. Six of one, half a dozen of the other. Avere le mani in pasta.