I sent out 6 cover letters and reusmes to the federal district judges in my hometown (Dallas) and got 6 responses for interviews. I sent them out shortly after Dec. 1, so you might not have as good of luck, but definitely worth a shot. IMO, the hometown will be your most fertile ground for itnerview, espif you go to law school in an area where there are a lot of law schools (e.g. Boston).
There are two real questions you need to answer. 1) Do you want to do a trial court or an appellate court? and 2) Where do you want to work?
Your interests will dictate the answer to the first question i.e. if you want to do litigation, a trial court is probably better, if your interest are more academic, or if you want more of an intellectual challenge/having to answer hard questions of law, an appellate court will be better.
Send cover letters to all federal district court judges in 1) your hometown, 2) the city/surrounding area in which you go to school (i.e. if you go to school in Boston, send letters to Fed District of RI, NH, as well as Massachusetts). Send some to the circuit judges in those areas too (these will be harder to get, obviously) if you want appellate work. It is not uncommon to send a letter/resume to every judge in a district. Sure, their offices are right next to each other and they will porbably notice that you sent them all the same letter. They know how it works, so this wont be a problem.
Also send some to the state supreme and appellate courts, though makre sure they are in session over the summer.
Addresses can be found on the internet usually. Each federal district has a website, as does each circuit. I'm not sure about the state courts.