“If you want to do anything new you must first make sure you know what people have tried before.”

This saying is not found in any source before 2010, when it was posted by The Ignorant Fisherman at Free Republic http://www.freerepublic.com/focus/bloggers/2451682/posts on 15 February. The language is not that of Washington or his time.
Misattributed, Spurious attributions

you ask. "Well, I'll get more," he says. Just as at cricket, you get more runs. There's no use in the runs, but to get more of them than other people is the game. So all that great foul city of London there, — rattling, growling, smoking, stinking, — a ghastly heap of fermenting brickwork, pouring out poison at every pore, — you fancy it is a city of work? Not a street of it! It is a great city of play; very nasty play and very hard play, but still play.

The Crown of Wild Olive, lecture I: Work, sections 23-24 (1866)

1960's, Conversations with Samuel Beckett and Bram van Velde' (1965 - 1969)

Source: A Prayer for Owen Meany (1989)

"Olympic boxer Marlen Esparza is ready to fight animal cruelty" http://nbclatino.com/2013/01/29/olympic-boxer-marlen-esparza-is-ready-to-fight-animal-cruelty/, NBCLatino.com (29 January 2013).

Twitter post, https://twitter.com/neiltyson/status/350753146445893633 (2015-03-03)
2010s

Source: Red Dragon

Source: The Perks of Being a Wallflower

Source: Matilda said, "Never do anything by halves if you want to get away with it. Be outrageous. Go the whole hog. Make sure everything you do is so completely crazy it's unbelievable...