1. In rare instances, it can happen after a federal clerkship, but those candidates tend to clerk for two years (not one), and the judge is heavily involved in helping to make it happen. Again, I think DOJ would look more favorably on a two-year district court clerkship than a two-year circuit court clerkship, because they are looking for more trial-ready candidates.

2. Candidates become a prosecutor with another state/federal agency (DA's office, SEC, etc.), gain experience, and the top performers get recruited by the DOJ to fill AUSA openings.

3. Gain trial experience in private practice, then apply for AUSA position (which sounds great in practice, but few attorneys actually get significant trial experience at biglaw).