The venture of taking street portraits is nothing new. It's been done before and yet it continues to be a dependable way to strengthen your comfort zone for approaching complete strangers. I personally done it myself and if you wish to press yourself even further to be less shy about it, there's a neat Flickr Group and project surrounding the concept and the rules are very straightforward:
The idea of the 100 Strangers project is to challenge you, to learn over time and develop your own style in approaching strangers and taking portraits of them. It is intended that you take the time, patience and effort to develop the skills required. The challenge is in no way considered a race to photograph 100 strangers. Get to know your subjects. Who are they? What is their life like?