1. Smile. Genuine one.
You'll know it yourself if the smile isn't genuine, then back off.
2. Engage in conversation - not necessarily about the camera, although having a camera or gear that catches attention more than makes up for it. (Usually it makes *you* the interesting person)
3. Be clicking other things, not just the portraits.
4. Someone mentioned Timing above - very important. I find myself happier taking snaps early morning or early evening - And, people more willing as well.
Try during times people might want to be clicked.
5. The 100 strangers project is a good-one to quell fear n boost confidence.
(I had tried it for a bit and the above is the common things to when I got successful street portraits)

Ken and Poison du jour make excellent points about people skills and those have worked for me as well.

Also, someone mentioned that "'third-world countries' like India" apparently it's easier to photograph. No, its not. I live in India and I have faced people saying no, people refusing to let me shoot near buildings etc., The world's a lot different from even 5 years ago, let alone ten.
And the dslr-toting 'enthusiasts' with rotten manners or phone cameras have over-run the earth.

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