I started doing this just a few years ago and found that, like most things, practice and experience helps. I am very much still learning!
You need the courage to stand there and shoot, and you also need to be able to pick a location with good light and have the ability to recognise an attractive pattern formed by the people/objects within the shot. I tend to find a spot and pre-focus, relying on depth of field to get a result. I typically shoot with either a 28mm or 42mm equivalent focal length and generally have to take several shots in order to get one decent result.
Sometimes I will ask if I can take a person's photo, and mostly they agree, and actually those shots have sold rather better than those taken without the person's consent. Looking back over my sales in the last 12 months street shots do not form anything like a majority, but they do sell.