Thanks 
Well this is my first time taking such pictures so I'm not an expert, so what I'v learnt I'v learnt today.
When approaching you have to be confident in your approach, even in terms of your tone of voice, your body language needs to non-threatening (smile) yet open, so don't face your subject directly with your body as you open the potential subject, once you have built a little report (after about 5 seconds) you can then face them head-on.
A little teasing of the opposite sex can help build report also, but needs to be used with caution.
In terms of conversation you need to give yourself and them a genuine reason for taking their picture, are you trying to build a portfolio? or complete an art project? etc. etc.
In terms of negative experiences, the worst I received was a smile and a polite "No thank you".