Practice, or at least read about, time management. In the Getting:In section you will find a few links to the DoE handbook for nuclear operators, those can be good to GLANCE at. I emphasize glance so that you get an overview of the information but can still easily solidify the knowledge how NNPTC wants you to, rather than how it is with the DoE.
More than anything though... time management. The subjects are not that difficult in terms of concepts but it comes quick, and it is completely different from what public schools taught you. In otherwords while most math problems in school ended in nice little solutions like 1+2x after the correct application of a few different mathmatical concepts, dont be surprised when "simplifying" in nuke school leads to a more complicated looking equation :-)
If you start paying attention to detail in your everyday life now, and try to get into good time management habits, I promise you will be one of the ones having fun on the weekend, and spending the evenings relaxing, rather than constantly stressing out about homework.