State laws usually protect employers when giving references, so long as the information is true. A key question that most new employers will ask is whether the company would re-hire you.
Ask for a reference letter when you are leaving. You have some leverage because companies don't want to have any problems with workers who are leaving. For example, if you file for unemployment or make a complaint with a government agency, it takes time for the company to handle it, even if you won't win.
Try to get as good a letter as possible. Offer to write a draft for them to look at. There are websites with sample reference letters (see the links in the Resource Box on this page).