Also as a note of encouragement, remember this. The Arminian approach to witnessing is one of anxiousness and second guessing after the fact. We've all been there at one time. I should have said this, I should have done that. If only I would have mentioned.............. The Reformed Christian knows that he is responsible to present the Gospel clearly but also knows it is the power of God that will save His elect. That takes alot of pressure off of us!! We are called to be a faithful witness and the rest is up to God!!
I agree with Puritan and his response. It is a skill to be honed. One thing I like to do after the initial conversation, is to just ask questions when I see the person to keep them thinking. How do you find harmony between the old and the new testaments? Israel were God's chosen people, did He switch to plan "B" for salvation and let people choose Him from now on? Does this mean plan "A", the Law, was a failure? Does God fail at things He does? Things like that to keep them thinking.
