I have found that having samples of your work and the types of poses you want can help - especially in the situation of men asking women to pose, they often think there is a sexual element or potential stalker lurking. Being able to have a portfolio of pictures which represent what it is you want helps (and hurts, since they often think they need to mimic what you have shown them). In the end, asking politely and taking "no" for an answer will go a long way. Also, give them a business card (something stalkers don't often carry) and tell them if they email you their address, you will mail them a print (and actually do it!!) - that way there is no pressure for them to decide at the moment and they do not have to surrender personal information until after they have seen how you act.