Like many of the commenters to Eugene's post, I agree that would-be federal court clerks should take civil procedure and basic criminal law and procedure. You will get a lot of cases in that area. Less obviously, I think it's very helpful to have taken legislation and antidiscrimination law.

Most federal law is statutory law. Moreover, the recent trend is towards enactment of statutes that are more complicated and detailed than those of the early New Deal-era regulatory state. A good legislation course can teach you a great deal about statutory interpretation that will help you deal with cases. In addition, if you become a litigator, it will be very handy for your later career in practice.

As for antidiscrimination law, Title VII employment discrimination cases are a large part of the docket in many circuits. Title VII cases are very different from constitutional Equal Protection Clause discrimination cases; it's much easier for the plaintiff to win under Title VII, especially in cases where's there is no direct proof of intentional discrimination by the defendant. So you should not assume that you have learned what you need to know about antidiscrimination law just from taking an intro constitutional law course.

Finally, although it won't necessarily help you with the task of writing opinions and bench memos, try to read up on the War on Drugs. Drug cases are the lion's share of the federal criminal docket and you will probably see a lot of them as a clerk on most federal courts. Indeed, the majority of all federal prisoners are incarcerated for nonviolent drug offenses. Even if you aren't categorically opposed to the War on Drugs like I am, you may still be troubled by the fact that we are incarcerating smalltime drug users and dealers for extremely long sentences, and by the reality that the War on Drugs has led to extensive serious civil liberties violations and to military-style police raids that have killed or injured numerous innocent people. Although I don't agree with all of its points, I highly recommend Albert Gross and Steven Dukes' America's Longest War as possibly the best reasonably short summary of the War on Drugs and the great harm it has done to our society.