I had the same problem when I tried to PR some Aristotle texts. Way too dense...
I believe the best thing to do in situations like that is to read secondary sources that explain Nietszche's basic concepts in a more understandable way. Everything Nietszche wrote is a working out of these basic ideas in full detail and if you don't understand the basic ideas, then the long passagework of his won't help you any. So as part of your previewing I'd recommend reading some background material on Nietszche.
