1. Make them want more
It's really important that you know, which are your intentions with the product or service you're giving for free. For example, if you intend to give a software, for free, and then ask people to pay for it, they will just feel offended. Why paying for something they got before for free? In this particular case, my advice will be to use trial periods, various versions of the product (Free / Paid) with different features.