Upon arriving at the bus or train terminal in a particular city, immediately make note of the departure schedules for outbound transport. Unless you like getting stuck in a particular city, or being pleasantly surprised with a bounty of no-choice free time on your hands, knowing your exits is a very good idea.
You can never have enough carabiners increasing the utility of your pack.
Don't bring too much. This is what I took to Mexico, and I should have taken less. Get used to wearing dirty pants. Bring lots of innards and layered outards. Travel more in spring and summer (it's more fun anyway).
Make sure you're up to date. Remember that some of them take 6 months to develop full immunity.
Bring a good pair of boots and a cheap pair of flip flops. The two will carry you in good stead.
Try the street food. It's generally safe. The two times I got sick in Mexico were caused by restaurant food. My belief is that dirty spaces, poor refrigeration, and shady re-heating policies are much likelier causes of foodborne illness than simple street foods that usually contain on-the-spot deeply fried meats, fried tortillas, and occasionally fried cheese.
Two things of note.
Traveler's Checks. Don't bother, they're a pain in the ass to convert, oftentimes are denied, and you get shittier rates. Bring a few just in case, but ATMs are a lot more convenient.
Big Bills. The quantities of money handled regularly by vendors is usually limited to small bills. In Mexico, you'll have trouble breaking a 200 peso bill, or roughly 20 bucks, at a time, and occasionally get screwed by an ATM dispensing 500 peso bills. I had really good luck with Scotiabank ATMs, which tended to break large withdrawals into 100 and 200 peso bills, and never 500s.