In my experience (granted, MD/PhD interviews involve more days of missed class):
1) work it out with teachers before you end up with interviews early in the school year (had to miss first week of classes and upset a teacher by missing so many classes)
2) don't take 20 credits (or maybe even 18 for that matter--especially if they're honors or graduate classes)
3) don't schedule them back-to-back-to-back during exam weeks (unless you want mono)
4) get applications/interview dates in early (so that you don't have to deal with this for two semesters)
5) don't spend energy on a back-up plan until later in the year if you haven't been accepted (especially if you have interview offers by September)
6) tell your PI if you are still doing research (lots of interviews + make-up exams + mono does not equal happy PI)
7) airplane rides = good study time (book reading, essay writing, math proofs...)
My GPA got rolled last semester due to my interviews. Depending on how many you go on and where at (e.g., driving vs. flying), performance in class will inevitably drop. I missed about 12 class days for interviews, which comes out to almost two and a half weeks. It's hard to make up that time IMO.
That said, bring your work with you to interviews and try and get as much done as possible. But make sure you enjoy the trips and explore the cities/schools as much as you can. It's not very often that you have a good reason to travel across the country and see a whole bunch of new places.