Pray for the Holy Spirit to guide you. He knows the person behind every door. God has promised to be with you as you do this. God loosened Zacharias’ mouth, and He can help you talk to a stranger.
Compliment the person. It could be the person’s home, his shoes, or his stereo system that you heard from fifty yards away. Just be careful about compliments to the opposite gender.
Look for the other person’s areas of interest. If that person has a Cubs cap on, talk about baseball. If he is wearing a Raiders jersey, offer to pray for him (only kidding, just offer up a silent prayer instead).
Read broadly. Not while at the door, but reading on areas outside your own personal interests will give you more areas to talk about when you first meet a person. Keeping up on current events will also give you things to talk about.
Ask open-ended questions, and listen to the responses. Questions like, “What brought you to the neighborhood?” can open up avenues of conversation and may even give an insight into his spiritual condition. It’s amazing how openly people will share their problems when you show a genuine interest in them.