I've had several friends backpack throughout Mexico and Central America. Some in a group, a few alone or in pairs. One of my oldest and closest friends went from Texas to Guatamala about 10 or 12 years ago, and he said the Mexican people were some of the friendliest people he's ever met. He said things were a little scary across the border in a few places, but once he got a couple hundred miles from the US border he didn't have a single problem. It might have helped that he spoke Spanish. He told me stories of families with next to nothing trying to share their last frijoles and tortillas with him and the girl he met down there. One older guy that owned a farm and was fairly prosperous took them to his farm, let them stay as long as they wanted, and threw a big party for them when they were leaving.

A couple of people I know had minor run ins with the police, but they just paid a small "fee" and went on their way.

*Crime is always worse in tourist areas.
*Know where you are and what the culture is.
*Remember you are operating in their culture, their system, not bringing your with you.
*Listen to your inner voice (instinct, whatever you want to call it)