First and foremost, this post is not meant to offend anyone, it is meant to give you a different perspective.
I have been witnessed to in many different ways, from people holding up signs of pictures of "hell" to being shouted at and being told I'm a sinner and need to repent or risk eternal damnation, to the now very popular list of questions "have you ever stolen, have you ever lied... etc". I am here to tell you that almost all of these methods turn people off. Let me tell you what goes through the mind of the average person who is approached by one of these "you are a sinner" method witnesses... "Oh crap, now I have to deal with this idiot" They are turned off from the begining.
Being a witness is not so simple that you can walk up to someone, ask/tell them that they are a sinner, tell them that god will forgive them and move on... The average person thinks of this as a minor inturruption in their day.
Some guidelines...first and foremost stop telling people they are sinners. How would you like it if someone walked up to you and told you that you are a terrible person? Telling people that they are inherently bad people right off the bat is a great way to make them tune out to everything you say after that. An alternative? Walk up to someone and see if you can ask them a few questions...
"Hi I'm John, Would you mind if I asked you a few questions?"
"Sure"
"First, Whats your name?"
"Dave"
"Nice to meet you Dave"
"Dave, would you consider yourself religious?"
Now here is where you actually have to start using your brain...
"No, I'm an athiest, or yes I'm Hindu (or whatever)?"
"Would you mind telling me a little about why you don't believe in God?/Why you believe in your religion?"
"Sure, I dont believe in god because of 'X'"
Listen to what he/she has to say. People actually do have reasons for what they believe. Then I usually tell them why I am a Christian. "I'm a Christian because of Gods amazing love for me, not because I am a sinner, not because I'm worried about going to hell, but because God created me I appreciate the fact that I'm even alive, and for that God deserves my worship. I honor God by having a relationship with His son Jesus Christ."
Then I follow that by asking them if there is anything really tough going on in their life.
"Dave, is there anything going on in your life that is really tough to deal with? Stress from work, an upcoming test at school, a girlfriend problem, because I would like to pray for you."
"Right now?"
"If you want, or if not I can pray for you later."
"Well, I don't think so.../Yeah work problems (or whatever)"
Either pray for him, or If he isn't interested tell him that it was really nice talking to him and you hope he has a good day.
Voila! You have just witnessed to someone without turning them off to God. You didn't even have to call him a sinner.
The essence of God is his love. Grace is important. There is no doubt about that, but what people need is to see that God loves them and wants them. Get people thinking about Jesus, let them see God's amazing love in you, and repentance will follow. Tell people about God's love and perfection and they will soon realize their own shortcomings and imperfections.
Love in Christ,
John