In general, I would say good people make good missionaries. You know, just like good people also make good husbands, good fathers, good wives and good employees. In general, the better a person you are the better you will be at being a missionary, as well as a thousand other roles in life. You will always be better the more loving you are; the more joyful you are; the more peaceful you are; the more patient you are; the kinder you are; the more gentle you are; the more faithful you are; the more self-controlled you are. The more the Holy Spirit has molded you into the likeness of Christ, the more effective you will be at being his witness.
But here are three things you can start with for being an effective missionary:
- Have a close, abiding walk with Jesus and his Word.
- Have God's view of the people you're trying to reach.
- Have a good approach/strategy.
Also, have them in that order, too--not the other way around.