We all suffer with this to some degree, and some people feel so self conscious, and such a tit that they wont even kneel down in public to get a better viewpoint/ angle for their shots. It's something you need to overcome, and the best advice I can give you is try to be friendly, outgoing and open in your approach and you'll mostly be well received by people. There's always the sneaky approach if all else fails.
I'm trying to find one of my old snapshots which is quite amusing and illustrates people's attitudes to photographers contortions quite well.
