Take an in-depth, detailed emergency medical course. The more detailed the better. We have been extremely fortunate in: A) not having a lot of injuries or serious illnesses, and B) being close enough to civilization to get help in time for the ones that have popped up. I would not count on having the same luck if I were you! I did not have a clue when we left just how isolated you could be out floating around on your boat in Mexico. Also, I did not realize the fact that not every town in Mexico has a doctor, a pharmacy or a clinic. And third, I did not realize how slow a sailboat travels. When there is a problem (like when Jack dislocated and broke his toe) it is a little too late to realize that you don't have a clue as to what to do and the book that tells you is back on the boat, plus you have to READ it first. It is ridiculous to think that in the middle of a true emergency you are going to be able to read a book, understand it and follow it's directions. Thankfully, Jack's toe wasn't a real medical emergency, but it still makes you feel helpless and stupid. Get educated!
Take some Spanish lessons. I thought that I would pick up Spanish as I went along. WRONG! I am not a quick learner with a foreign language and evidently there are lots of others like me. Just about every cruiser I have met down here who did not know how to speak Spanish before they left, still do not know how to speak Spanish now. The cruising lifestyle moves you from town to town, where you have the same conversations over and over with different people, but you never get a chance to form relationships with them. Therefore, I am really good at asking "Donde es .....?" and I am one hell of a good orderer in a restaurant. Those are about the only conversations I have. Unfortunately, I have missed out on a lot of meaningful conversations with some wonderful people. There have been many times where I would love to communicate with someone or understand some nuance of a situation that is completely beyond me with my limited Spanish. Sure, you can easily get by with mime, a smile and couple key phrases, but Mexico is filled with wonderful, kind people who are worth getting to know. Learn some Spanish and give yourself a chance to really experience Mexico. (And yes, I did take two years of Spanish in high school, but that was a long, long time ago.)
Take the ham radio test and get the General license at least. I've talked before about the SSB radio and how much I appreciate it. It is a wonderful tool for cruisers and provides many benefits. Not every SSB radio net requires a ham license to participate, but some of the best ones do. I managed to take my ham test while in Mexico, so you can get it done after you leave. However, that required being in a certain place at a certain time of year - something that is not always possible on a boat. If you are planning on being gone a long time and travelling in remote areas, I would strongly suggest you have an SSB radio and a ham license. Get your license now, while you have a car and can easily get yourself to a testing center.