In a word? Accuracy.
Casting my mind back a good few years, that's what screwed most people with their nav exams. You need to accurately measure things like bearings, distances etc as the answers are deliberately close toghether to catch out the slapdash types.
eg: If a bearing is roughly about 080, the answers wont be 070,080,090,100, they'll be 080,082,084,086. If you measure it accurately, take some care, you may find it's 082 or 084, not approximately 080. So if you just think 'Oh, 080 is about right' - you'll get it wrong.
So take your time and be systematic and you'll be fine, most of the Nav exam is a plog (as is the IMC exam come to think of it).