Learn some Spanish, even if you plan to hang around resorts. A little will go a long way. A few food words will come in handy, too (cerveza = beer, for example).
Mostly, you'll be using pesos. Take a moment to look at the notes so that you won't fumble with them later. 20-peso notes are blue, while 50 notes are reddish. 100 notes are red. 200 notes are green, and 500 notes are brown. 1,000 notes are purple.
Mexico is generally a fairly healthy country for travelers. However, there are a few caveats. The worst threat to traveler health is traffic; however, that's something true in many countries. Be extra careful on the road, either when driving or on foot. Generally, it's also best to drink bottled water; avoid consuming tap water.
Certain places near the border are suffering from war-like violence, but most of the country is not. Travelers mainly need to be on guard for petty theft, especially in larger cities. That includes purse snatching, pickpocketing and ATM scams. Don't flash your cash, and try to use ATM machines only at banks and in larger buildings.
Respect the law. If driving from the United States, get Mexican insurance. Obey speed limits. Never under any circumstance possess or consume illegal drugs. Not only will you risk time in a Mexican prison; you would be supporting violent criminals.
