My aunt and uncle traveled extensively in Mexico for many years. Everything they told me backs up Blast's advice. Many of the people there have gotten meaner, just like the ones here. But if you listen to them and make deals with them for small jobs, most of the time they will be honored.
For instance, if a kid offers to "watch your car" while you're eating lunch, make a deal with him to do just that, and make a bargain with him before you enter the restaurant. At least in the past, the locals figured if a local kid made a deal with a tourist to watch his car, someone was making money and they would honor it and everyone would leave the car alone. If the tourist was rude, and brushed the kid off, the car would be a shell in half an hour.
Mexico is a poor country, and things like luggage and car parts probably have considerable value.
Keep some bribe money tucked right under your license so you don't have to fumble for it.
Sue