Depends on the situation.
Are you talking about while at work, or in the street either walking or standing while someone preaches or knocking on front doors?
At work tak a Christian book you are reading and leave it out on your desk. Be willing and ready to talk about it. Lee Strobels 'The case for Christ' is one good book.
In casual conversation ask, 'What do you think is the Christian responce to ........' and see where the conversation goes.
If standing while someone preaches in the street, simple ask, 'What is your opinion?'
Door knocking, an opening question, ' If you stood before God and he said'Why should I let you into heaven?' what would you say?'
Loads of openings if you get a reply.
We had a tract written with on the front, "o thanks I'm not interested!" because so many peiple would say that. This tract sometimes got a laugh and a conversation.
