I wish I had known how difficult long-term fruit really is.
On a short-term, you may go to a responsive area and see numerous decisions for Christ. But among less responsive people groups, missionaries often struggle with spiritual infertility. Sometimes those who make a decision may not follow through. On a short-term, you saw people pray for salvation and then you were back on the plane. But as a long-term missionary, you may discover that this "convert" no longer even wants to see you.
If you want to see people saved, you need to spend time with non-Christians. But we were amazed at how much of our time was tied up in fellowship with other missionaries, people in the church, other national pastors, and friends who visited. We started to ask, "When was the last time we saw a non-Christian?" Then God led us to make contact with a pre-Christian every day. As part of this we invited families over for dinner once a week. Though few outsiders would come to a church meeting, no one turned down a dinner invitation!
Missions is sometimes a huge challenge. During our darkest days I would read Hebrews 11 aloud twice a day and pray for the faith to keep going. One day I continued into the next chapter and read Hebrews 12:11, which says, "No discipline seems pleasant at the time, but painful." I wondered if the Lord was disciplining me. Had I been disobedient? Then I read Hebrews 12:7 "Endure hardship as discipline. God is treating you as sons." Then I saw it. The hardship is something that God brought into my life. Then I reread verse 11 and exchanged the word discipline for hardship. "No hardship seems pleasant at the time, but painful. Later on, however, it produces a harvest of righteousness and peace for those who have been trained by it." Like a good marriage, effective missions takes work. But it's worth every bit of it, too.
I wish I had known how nationals view the economic status of missionaries.
My standard of living is drastically lower in a developing country. But no matter what level one lives at, the fact remains that you will always be considered wealthy in the eyes of the nationals. And in fact, you are wealthy. In our local currency $2,000 US dollars would equal a million, so almost all the missionaries are "millionaires" here. And even if you find it hard to live on the support you receive from back home, you still had enough to pay a ticket to come here, whereas your everyday national could never pay a plane ticket to visit another country.
Also, one encounters the common idea that it is the duty of those with more money to distribute it to help others. So the nationals will never look at someone who has more money than them and lives at a higher level than them, and drives a car, as someone who is making a sacrifice. Don't expect to be congratulated or thanked for the sacrifices you made to come. Americans in particular (and I speak as one) seem to have the idea that we must be constantly affirmed. Better get over that before heading overseas.
Another fallacy is that nationals should applaud that you have "sacrificed all" to bring them good news about Jesus. The reality is that they probably won't give a hoot! Many missionaries go with the idea that they should be "appreciated" by the nationals for the sacrifices they have made. And of course we cannot serve God if we are not appreciated! Pray to be humbled now, before going out and being humbled overseas. Of course, depending on your job (doctor, nurse, well-digger), you might be better received than just a general evangelist. Or if you come to work a specific job by the church, they will be more appreciative than non-Christians. But I guarantee there will always come a time when you will feel that you are not "appreciated" (whether by the church, the heathen, or even your own colleagues).
I wish I knew how to deal with conflict.
When you want a job you usually put on your best for your prospective employer; it's like a first date, you hide all the bad and accentuate the positive. Unfortunately, I discovered after two failed attempts to work with missions agencies, this not a good way to "get married" to a sending organization.
I fell in love too fast, accentuated my and their positive points, and didn't ask the critical question, "How do they fight through a problem?" Neglect to do this and you could get seriously hurt.
When you know how a spouse, boss, friend, co-worker, pastor, or mission agency resolves conflict, you will know your chances of being able to have a long-term relationship with them. Nice Christians who resort to threats, gossip, slander, lawsuits, giving the silent treatment, bullying etc. don't tell you up front this is how they deal with conflict. You have to know them well before you commit to a long-term relationship. So find out how they fight before you sign up.
Ken Sande's ministry has an abundance of information how to deal with conflict biblically. Reform your own conflict resolution methods first, then look for other peacemakers you can work with.
(These last three posts were taken from www.askamissionary.com - a great , insightful resource for all things missionary!)