Take a picture of the first few pages of your passport and then email them to yourself. That way you will always have the information available in case you lose your passport.
TRY to communicate in the local language. Even if you fail miserably people appreciate the effort.
Wear comfy clothes for the flights. Some can be long and boring or just delayed, might as well be comfortable.
Get a money belt but don’t put all your cash in one location.
Be flexible. Things can, and will, go wrong, expect it and if it doesn’t well, great.
If you are from the US realize that the rest of the world travels at a different, more relaxed pace. Be there on time for your train, bus, whatever but don’t panic or get angry when it is 15 minutes late. Think “Island Time”.
Book Early.
Keep and open mind; you are not in Kansas anymore.
Never use a credit card at a Pemex station.