in regards to selective service, if you dont register you dont get financial aid or federal jobs, pretty much anything related to the feds. you have until 26 to do so. also, as part of Bush's great education reform, No Child Left Behind, any school that is to receive federally funding must provide info on students and be allowed on campus. however, it is at students digression to provide further info, sign up or register.
regarding jury duty, my friends missed a few, mostly because of moving, and he hasnt been arrested, fined or detained yet. in california, you can mark quite a few options to avoid serving it. ive mostly chosen, "if i dont work, i dont eat," box and sent them a letter from my employer and its worked everytime.