I was an atheist most of my life (until a year ago). The things you DO NOT want to mention are as follows:
1) Hell and eternal damnation: This is the worst argument you can use on an atheist. In most cases, he will actually resent you for implying that he will be eternally punished for not believing in your God. Warning him about hell is the best way to turn an atheist against you (just spend time in one of the atheist forums, if you don't believe me).
2) Scriptural evidence: Atheists don't believe in the Bible, so it only hurts your case to try to use scriptural evidence. If the Bible is true, and if God really created this earth and everything in it, then you should have plenty of truth (outside of the Bible) to talk about.
On the other hand, if you really want to convert an atheist, try this:
1) Listen to him: Don't just fake it, really get in there an listen to what he believes. Don't try to attack what he says. Let him get it out, and then ask more questions. Look for all of the virtue in his beliefs. It is likely he believes in truth, in his fellow man, in fairness/justice, and in love. Above all, you HAVE to show him respect.
2) Expose his pain: Most atheists ignore their own pain. Many don't even know it's there. Pain from isolation, failed sexual relationships, even simple selfishness. Ask about his life, his past... look for where he hurts or where he may have hurt others. Remember, this is NOT about exposing sin, this is about exposing pain. Pain comes from sin, but an atheist only speaks the language of pain right now - so don't go Christianese on the guy with all your "sin" talk. Get him to recognize the pain, and you will have opened the door to begin calling it sin later.
3) Show him your glory: Live your glory. Let Christ shine off you. Atheists talk about an "expanded worldview", but the reality is they have no idea how isolated they really are. They are drawn to Christians who reflect Christ's love, and they will respect you more for the love you display than for your intellectual discourse or argumentative rhetoric. Truly being his friend will do more for you than engaging him in apologetics.
Anyway, you'll notice that the best way to engage an atheist is to go after the heart. Stay away from the "Christian talk" until he asks you directly to share your beliefs with him. By then, hopefully you will have earned his respect. Good luck!