I came across an evangelical blog that had a list of suggestions up for witnessing to Mormons. I think it was a pretty good list, but I would like to add one more thing for Orthodox Christians who want to witness to Non-Traditional Christians such as the Mormons--make sure you understand Mormon doctrine as Mormons understand it.
Mormon doctrine holds that there is only one eternal God, or Godhead (Father, Son, and Holy Ghost) and that there are many gods, but they are gods by grace, made so by God himself. But we hold that there is only one source of worship and power and knowledge in the universe-–God (or the Godhead). No matter what exalted beings are called, we will always be subject to the one true God who made us what we are.
And that "shocking" quote about God once being a man isn’t such a stretch when you realize how we believe the scriptures which say that we are His offspring, created in His image, and that He is our Father. That means that we believe we are the same species or essence of God. So thinking of God as an exalted human being isn't such a stretch. Just as Jesus was once on the earth as a man, but was still fully divine and fully God and is now resurrected and glorified and exalted, we hold that God the Father also could have had a mortal experience, but there is no doubt that he is glorified and exalted. Remember also that Jesus was still fully God while he dwelt on earth as a man. We are "fallen" man, although we have hope of overcoming the effects of the Fall through the atonement of Christ, who intends to make us divine like He is. Now, if you've only asked the LDS missionaries about this and you felt like you didn't get a sufficient answer, just know that 19 year old missionaries aren’t the most informed in Mormon theology. They are sent to bear witness of the restoration, but they're not necessarily trained in theology. Actually none of us are, but some are more informed than others.
Last thing--about becoming gods ourselves. We literally believe the Bible when it says that by God’s grace we receive His glory (John 17:22-23), or as Peter said, we become partakers of His divine nature (2 Peter 1:4). We do not believe that it is only figurative. We believe the Bible is literal when Paul says that we are made joint-heirs with Christ to all that the Father has (Romans 8:15-17; Revelation 21:7; 1 Corinthians 3:22). God, by His grace, makes us divine through the atonement of Christ, which breaks down all barriers between the Creator and the created--making us “at one” with God through the at-one-ment. We believe, as Christ prayed in His intercessory prayer in John 17, that through his grace we can become “one” with God, as Christ and the Father are one. We will share in their relationship and, again, to quote Peter, fully partake of their "divine nature"-–thus becoming divine ourselves.
The Westminister Dictionary of Christian Theology (not a Mormon publication) says: "Deification is for Orthodoxy the goal for every Christian". Almost stating LDS belief exactly on the doctrine of deification is the great Christian defender C.S. Lewis, who wrote: "The command Be ye perfect is not idealistic gas. Nor is it a command to do the impossible. He is going to make us into creatures that can obey that command. He said (in the Bible) that we were ‘gods’ and He is going to make good His words. If we let Him–for we can prevent Him, if we choose–He will make the feeblest and filthiest of us into a god or goddess, dazzling, radiant, immortal creature, pulsating all through with such energy and joy and wisdom and love as we cannot now imagine, a bright stainless mirror which reflects back to God perfectly (though, of course, on a smaller scale) His own boundless power and delight and goodness. The process will be long and in parts very painful; but that is what we are in for. Nothing less. He meant what He said” (Mere Christianity.)
Thus, we believe we can be made into much more than we currently are through the atonement of Jesus Christ. He changes our nature. Nevertheless, we will still always be subject to, and worship only the Father, Son, and the Holy Ghost through all eternity. Plain and simple.