How do you prevent a hernia? Are any exercises a risk?

I never want to get a hernia. So, of the people who get them, what do they trace as the cause of it?

Can exercising cause a hernia? What about situps or crunches? What other exercises?

What about pooping too hard?

Is there anything else important to know about preventing such a thing?

I already read the article at eHow, so I’m looking for information beyond that.