There is a little yellow card they give out at the MTC for Spanish speakers. I suppose other languages have something similar. It has a bunch of useful tools on it for learning the language. Find someone who served a mission speaking the language you are going to learn, and see if you can borrow or make a copy of that card, then memorize that card. You might not understand what it all means, but when it clicks for you, you'll be leaps and bounds ahead on learning that language.
If you are in a ghetto area and a shady looking group of guys calls you over, trust me they don't want to hear the gospel. But they might want your watch.
Study the scriptures in depth. A lot of missionaries have this sort of contest with themselves to see how many times they can read the Book of Mormon through as a missionary. I finished it only 2 or maybe 3 times. In my first year as a missionary I only read it through once. Instead of rushing through I took copious notes, studied meanings, tracked cross references etc...I was able to fill 3 study journals as a missionary, and they have been a huge blessing to me ever since. God will reward you with more learning the deeper you dive, and the more you record of the revelation you recieve.
Develop your talents wherever possible. You are actually smarter and more capable as a missionary than you have ever been before. Your capabilities and capacities are expanded because you carry more of the spirit with you and it will amplify your abilities. Improve your talents, and try to use them in ways that will benefit the work and you will see amazing things happen.
Take a nice camera, a digital one if there are going to be cyber cafe's or other places to download your pics. Take a nice camera, don't carry it everywhere with you.
Take a bazillion pictures! You have no idea how much you will cherish those memories later. Even the dumbest things at the time can be so much fun later on.
I was prepared for a mission to be hard physically. I was not ready for it to be hard mentally. I thought I would be happy all the time, not a care in the world. The truth is the mission will push your emotional and mental limits just as much, if not more than your phsyical limits. Be prepared for that, and you will be fine.