Nothing magical occurs when a person moves into a cross-cultural situation. Arriving on foreign shores, a new missionary doesn't suddenly morph into an evangelistic go-getter.
When we come across people who are about to go onto the mission field, while standing in awe of their commitment to Jesus, our usual question to them is this: "What have you been doing here in this country?"
Whatever a person is doing now, back at home, in terms of working for the Kingdom, is most likely what they'll do overseas. If they never spread the Good News here, it's not likely they'll find it any easier elsewhere. If they've never started a church here, it's not going to fall into their lap in a foreign context.
So what is the best basic training for you, as one whom God is calling overseas? Is it years of seminary? Bible school? [God uses such people –remember, the apostle Paul had done the equivalent in his day–but that wasn't what qualified him to reach out to the Gentiles.] There's nothing to compare with practical experience.
Many missions or missionary sending churches, no matter their denomination or affiliations, have recognized that old-style missionary methods do not generally have much impact on the Kingdom in terms of new followers of Jesus. Teaching in schools, bringing healthcare and so on may provide extremely valuable help to the local people, and that may be what God has called you to do, but it doesn't usually result in new churches.
The most effective pattern being used around the world today is the multiplication of networks of small, organic churches meeting in homes. We know several mega-churches who start house churches in other countries. No matter their background, this is what sending agencies are doing overseas. Many missionaries today train indigenous leaders in these patterns. Relief of needs–feeding the poor, caring for orphans, may play a part, and may provide inroads into the community, but it's not the primary means of gathering the harvest.
Therefore, the most effective preparation for overseas missions is to do here at home what you'll be doing, or what you'll be training others to do, abroad. Start a multiplying network of simple/organic/missional churches. This practical, on-the-job training, coupled with learning about cross-cultural challenges and opportunities, together with some experience in other cultures will provide a good basic training for what you will do overseas.