People often ask how I approach strangers. The honest answer: slowly, and usually without the camera in my hand. I introduce myself, explain what I am working on and ask if I can come back tomorrow.
Coming back is the important part. The second visit is a different conversation. People have had time to decide, and the ones who say yes really mean it.
Three rules I keep
Always show the picture. Always send a print. Never publish a portrait that the person would not hang on their own wall.