Make Sure That You Know Your Own Faith Well
Know not just what you believe, but why you believe it! Most Christians know to some degree WHAT they believe, e.g., the tenets of the Apostle's Creed, but they do not know WHY they believe it. The cults logically, capitalize on this ignorance to great advantage! Their chief point of attack is usually: "Why do you believe such and such ...?". Few cultists, however, can confuse a Christian who is well-versed in biblical theology. 

As the late Dr. Walter Martin has said so appropriately: "The poison of cultism can be effectively combated by the antidote of sound doctrine". (The Kingdom of the Cults, p352). In dealing with cultists, it is the WHY you believe that is more helpful and important than WHAT you believe. In your preparation for confronting cultists (and other non-Christians), study the REAL thing, not the counterfeit. A good anology is when the banks train their people to detect counterfeit money, they do not let them study anything but genuine currency! They become so familiar with the genuine notes that they can spot a counterfeit immediately. The same is true of God's Word. We Christians must become so familiar with the Bible that we will immediately recognize Satan's counterfeits by comparison.