just be honest with them. Most people dont have the time or inclination to approach people if they think they're taking pictures of them. If you do spot them staring you down or they approach you, just say "Hi, yeah I did take a picture of you. I just think you're an attractive/interesting looking person and you were in a great spot and it would make a lovely photo. If you're totally opposed, i'll delete it, but if you dont mind then id love to process it. If you give me your email address, i could even send you a copy when im done."
You'd be surprised how many people respond to a bit of honesty and flattery. There will always be one or two, but dont let it put you off. In any case, after the first time someone dislikes the idea, you'll be over it and it wont bother you again. You're only nervous once.
