Andrew: I could write pages and pages on this, but I'll do my best to give you a somewhat-condensed version of what I think the most important things every young LDS man should know before setting off on one of the greatest adventures of his life. My situation was very unique in that I actually spent a month in the field as a full-time missionary before I ever received my call. If you want the details of that story, feel free to ask, but for now, it's not important. First, learn to get along with your companions! Some of them will be much easier to live and work with than others. Enjoy the ones which are easier to get along with, but learn from the ones that are more difficult as well. Communication is key to making the best of those situations. Communicate in a way that allows the spirit to be with you both. If you're not sure what to do, mention it in your weekly letters to your mission president, but don't take every little problem to him either. The lessons you learn as you serve with each of your companions will serve you well as you move forward with your life and have to work with people you don't like and also in your marriage. Do your best to follow the rules, but do not ram the rules down the throats of everyone around you. My companion in the MTC made my experience there a very negative one because he felt it was his place as our DL to force everyone to follow each rule to the letter. All it did, though, was create tension and resentment. Do not let your companions push you around, and do not let them hang their status as senior companion over your head. The only purpose for even having a senior companion at all is to maintain order and accountability. And don't do it to your junior companions either. Do not be afraid to work! Begin now to be up and out of bed every single day by 6:30 a.m. so it will not be as difficult for you in the MTC and in the field. Plan your days so that you're not stretched too thin, but do not be afraid to work hard, even if you don't feel like it. My trainer taught me how to work hard, but he also taught me how to work smart. The afternoons were often too hot to do much outside, so we'd try to stay indoors in the middle of the day and go out in the mornings and evenings. I'd also suggest learning some basic cooking skills. You may hope to be fed every night, but the reality is that you won't. The better able you are to throw something together, the better off you'll be. Do not, ever, aspire for leadership positions. This will be a time to teach the gospel to those you meet. Your mission is not a time for you to seak for and obtain leadership so as to gratify your self worth. Above all, and I'd say this is the most important thing...pray to learn to love the people where you are called to serve. I didn't discover the value of this until I was a year out, but when I did, it changed everything for the better. If you are blessed with a love for the people, everything else will fall into place, and you'll rarely struggle to find the motivation you need to work hard and to study. For me, the love for the people was so strong by the time I went home that it has never faded in the 13 years since my mission ended. Strive to have the spirit with you at all times, but also have fun and enjoy the whole experience.
Andrew, first of all, congratulations on sending your paperwork. This is probably the most important and best decision you've made so far in your life. You are doing right by the Lord, be sure of that, and many people will be truly blessed by your decision, including you.
J.D. has done a great job laying out the fundamentals of mission preparation in his reply. I wouldn't add anything, I couldn't put it better than he did. What I'd re-emphasize, though, is this: Do not, ever, seek leadership positions. I received this advice from someone before I left for my mission and it really made a difference. Seek to please the Lord thru your hardwork and willingness to serve, you will feel him close, and everything else will come as consequence. I made by best to no aspire positions and, interestingly enough, had all of them. And the truth is those things make no difference whatsoever. So really don't even think about it, just do you best and you will have the time of your life (so far).
Andrew: I hope you're still following this discussion (such as it is) because I thought of a couple more things yesterday that I think will be very helpful to you in the field. I hadn't been out in my first area a full day when my trainer and I encountered a guy who was very hostile toward our church. This guy tried to start a debate with us, but he hadn't got more than a few words out when my companion shut him down by telling him we were not there to argue with him or try to convince him of anything. We were there to teach those who were interested in our message. My point is: Don't ever allow yourself to be drawn into a debate or argument. Where conflict is, the Spirit is not. The other suggestion is to prepare at least 3 good talks and keep them with you at every meeting you attend. You never know when you'll be called on to give a talk, and you should e well prepared for it in advance. My wife went on her mission a few years later than I did, and she just told me this morning that when she was in the MTC, they told everyone to have talks prepared. When sacrament meeting was held, there was no set schedule of speakers; people were called at random out of the crowd to come up and talk. That should give you an excellent reason to prepare now. Please let us know when you receive your call and where it is you will be going.
Honestly, I don't remember anything about the meeting with my stake leaders prior to going into the MTC. Perhaps part of that was that it would have happened some 15 years ago, and part of it may have also been that my Dad was my stake president at the time, which wouldn't have made it a very memorable experience for me. I'm sure they're just evaluating your maturity and fitness for what is to come, make sure you're worthy of course, and gauging whether you need some advice on various preparations you can make to be better prepared when you do get out there. Honestly, it's nothing to worry about.
I'm going to follow the "teach them correct principles" method. So, a list of the most important principle and characteristics for preparing and being a missionary.
Your faith in the Savior and His gospel will fuel and enable everything else you do.
Recognizing the Holy Ghost
You must recognize the promptings you receive in order to follow them. You also need to be able to help others recognize the Holy Ghost testifying to them. Not everyone feels the Holy Ghost the same way, nor every time.
Charity and Compassion
This is the other thing that will motivate you. Remember the two great commandments: Love God and love your neighbor - all else hangs on these two.
Physical, Mental, Emotional, Spiritual. Long days of walking, constant rejection, hours of study, etc. It is worth it, but it is WORK and if you are doing it right you are working HARD. I learned to love the work and love working hard at it, I hope you will too.
Knowledge of the Gospel
My age was such that I got a full year of college in before I left and took as many Institute classes as I could fit it my schedule [btw Arizona State institute is awesome :)]. One of them was a mission prep class and we had a saying we repeated at the begining of every class. "You can't teach what you don't know any more than you can come back from where you've never been." Or, as the Lord put it:
D&C 84:85 Neither take ye thought beforehand what ye shall say; but treasure up in your minds continually the words of life, and it shall be given you in the very hour that portion that shall be meted unto every man.
The Lord can give us the words He wants us to speak, but the more they are in us already the easier they are to bring out.
With your companions, with your investigators. And forgiveness. Be slow to take offense and quick to forgive.
Obedience to the mission rules will bring power and safety. Having the Holy Ghost with you is the most important thing, and breaking the rules will offend Him and leave you to your own limitations. Remember that the rules, like God's commandments, are there to help you and protect you.
If you can develop, and keep developing, those characteristics then I have little doubt that you will be a great missionary.
Surely you've received your mission call by now, right? I'm still very interested to find out where you're going and how you feel about that.