I took the NLN in January and honestly I don't know if there really is a way to prepare for it. I sent for the old exams and the only thing that really helped me with those is to know the format and time restrictions. Lots of people I know didn't finish certain sections because they ran out of time.
I never ran out of time, thank goodness.
I think that basically the NLN exam is just a IQ test. They want to see your basic knowledge of math, science and vocabulary. I never took biology (got the Department Chair to sign me off) and I went straight to A&P. When I took the exam, I only had one semester of A&P and no micro yet and my score was a 130.
In our school, a 100 is passing, 115-123 gets one point, 124-129 gets two points, 130-133 gets three points and 134 and above gets four points. Is your school similiar? I have only met a couple people who haven't at least passed so most do well.
I would just brush up over the basics and say a prayer!