I currently intern at the DA's office. The steps you need to take at our office are unpaid/volunteer student clerk (where I am at currently), paid clerk, and then after you pass the bar, hopefully you are hired as an ADA. They have a point system that can up your chances of getting hired. For example, you get points for the total hours you intern there, some points if you have other legal experience, if you were in the armed forces, if you were employed in corporate america. The higher the # of points, the greater the possibility of being hired after you pass the bar.
I think it comes down to like any other job. For example, you put in the time at a firm and they like you. If you have more experience than the next guy, when you pass the bar, they hire you and not him.