Make sure you have a desire to want to report on Congress. Get up to speed on current issues facing Congress, particularly if you know the issue area for the position you are applying for. While CQ is behind a paywall and you might not have access, read as much as the public Roll Call site as you can before your interview. Be armed with as many story ideas as you can be so you can describe what you would jump into if you got the job. Think about reporting challenges you've faced in the past and how you have handled them and be ready to talk up some of the more interesting stories you've done in the past.