I took the exam a few weeks ago, and I would say the two things that really stuck out to me (for those who may be taking it in the future) were:
1.) The bad (sub 20) and very good (high 30's- low to mid 40's) beers were very easy to score I thought. If you know the style guideline (basically) of the good beers, it almost checks itself off. The worst beer is usually a lot about process flaws..... which leads to other problems like its appearance, its aroma, flavor, etc. It also makes it easy to give some advice. I found the 25-35 type beers harder to judge. Each had something that was pretty decent. Each had some things that were lacking. In hindsight, the one descriptor that I really wish I would have thought of at the time for those beers was "one dimensional." As I pondered my judging on the way home, that was something that occurred to me. Some aspect of the beer (malt, hops, etc.) was present and good but then something else was throwing the beer off like astringency, or harshness, etc. and it just left the beer lacking overall. They were not major flaws - but seemed to me like water issues, or maybe recipe balance issues, or perhaps simply yeast health and a fermentation that was not quite where it needed to be.
2.) The other thing I felt was challenging was coming up with quality, concise descriptors and adjectives. It is hard to naturally think up a useful adjective when you are under the gun. I felt like I could have phrased things better than I did. So, simply practicing writing out score sheets, and having some various descriptors that you have practiced using is a big help.
*I found it helpful to use the BJCP Checklist Scoresheet as a "studyguide" for descriptors.
*I found it helpful to write scoresheets on the classic style examples and then compare them to the sheets Gordon strong has posted on the BJCP site.
*Having entered quite a few competitions was helpful, simply from reading a lot of scoresheets of different quality.