The flight planning exam is one of the most straightforward simply because it requires very little memorisation of any facts. You will find about 15 questions will just be from the graphs in the CAP. In the exam I would recommend going through the paper and doing all of these first.
About half of the questions come from the various charts in the jeppesen manual. There shouldn't be much problem here, the plotting is fairly simple, and many questions just ask what the symbols mean. These can be obtained simply by looking on the appropriate legend in the jep manual. As with the CAP questions, do the chart questions all together to save time folding/unfolding charts.
Many of the remaining questions will also be answerable straight from the manual. About 10 questions in my exam were about the flight planning form - automatic easy marks. Probably the only formula you need to remember/derive is the point of equal time/safe return. This should stick after you have done a few questions using it.
Hope this helps to some degree, I think the most limiting thing with the Nav and FP exam is the time limit. Try and work as briskly as possible, do not struggle with a question that is only worth 1 mark for a few minutes. If you have time at the end you can come back to it. Make sure you have answered those that are worth 3/4 marks.