A few favourites...
‘Know then thyself, presume not God to scan;
The proper study of mankind is man.’
Alexander Pope

'I have seen these things in a shaft of sunlight.' T.S. Eliot

‘If your pictures aren’t good enough, you aren’t close enough.’ Robert Capa

'If you want reality, take the bus.' David LaChapelle

‘Somehow I was using the camera to make connections. I have a tendency to work out of a state of mind.’ Bruce Davidson

‘I never have taken a picture I’ve intended. They’re always better or worse.’
Diane Arbus

‘There was always a voice in me telling me to try, try and try again, and that bloody voice is still saying it, and it's wearing me out' Don McCullin

‘Photographers deal in things which are continually vanishing, and when they have vanished there is no contrivance on earth which can make them come back again.’ Henri Cartier-Bresson

'...to know ahead of time what you're looking for means you're then only photographing your own preconceptions, which is very limiting' Dorothea Lange or...

' What am I looking for? I can't tell you, I aint found it yet.' Les Paul

' it's easier to ask forgiveness than it is to get permission' Grace Hopper

' f8 and be there ' Weegee

'To me, photography is an art of observation. It’s about finding something interesting in an ordinary place… I’ve found it has little to do with the things you see and everything to do with the way you see them.' Elliott Erwitt

Then these from the Street Photography Now Project run by the Photographers Gallery in 2011...

'The real subject is yourself facing the world' Thierry Girard

'See the characters but create your own plot' Martin Kollar

'It might all come together for a moment and then just as quickly it is gone.' Narelle Autio

'Don't ask. Shoot fast.' Gus Powell

'When you hit that wall of utter frustration while photographing the street, when you are beyond tired and just want to give up, keep on walking - for another hour, or until the light goes entirely. Often the best photographs come when you least expect them - when you are the most exhausted, and the most emotionally vulnerable.' Alex Webb

'The distance between yourself and others should not be greater than your arm's length.' Christophe Agou

'Pick a spot, stay there for an hour and see what unfolds.' Polly Braden