Take full advantage of every opportunity you have. Introduce yourself to as many people as you can, go out of your way to ask staffers to coffee to get career advice, and look for ways to know what's going on around Capitol Hill at any given moment. There are a million briefings and luncheons that interns can go to where you can meet incredibly knowledgeable folks and learn aplenty. Especially try to give as many constituent tours as possible. You become a master of Capitol-trivia and when non-constituents (i.e. current government officials, foreign ambassadors, former staffers, etc.) want tours you can be the go-to person to give them a tour. I'm sure I don't have to explain why getting to spend several hours practically one-on-one with people who are already working in the field you want to be in is valuable.