Hi, I have already taken both exams and passed first time and if I can pass them anyone can!!
I used the Trevor Thoms (Pooleys) Navigation book and an hour with my instructor to go through the planning charts in the aircraft POH. As long as you understand how these charts work and what the meaning of TODA, TORA etc etc is then for the flight planning exam you'll do fine. For the NAV exam, as long as you can plan a flight accurately on the chart and Plog, understand the basics on radio Nav (ie what forms are available and how to use them), chart symbols (hint: use the charts symbol key!!), low flying rules and compass error/deviations, again you'll be fine!!
I really enjoyed those 2 exams actually as strange as it sounds!!
Good luck!