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I mostly just say it as it is - "I think this is a beautiful picture", "..that colour / light / etc. looks very nice right now, would love to capture this", "I love your outfit / bike / etc. - may I take a picture of this".
So I guess it's good to use honest I-statements, as they come across, well, honest - and the person gets the hint that you actually care about making a good photograph - and an analogue camera definately helps in this regard.
I'm pretty shy often times with strangers, but I've found just acting as if you're confident, and being nice and polite works pretty well.
In other words (two statements I read a while ago and remembered) -
"..just ask, most of the time they say yes", and "smiling really helps".
This is the most common one I get, really hard to reply without sounding creepy. E.g. 'you're interesting' Any tips anyone?