Congratulations on passing the MET, as for the Nav I found the exam easier than expected, BUT, you need to know your stuff, make sure you are clued up on lattitude/longditude reading, conversions of magnetic headings to true headings and visa versa (variation west, magnetic best and all that, and remember its the oposite way round for a conversion from magnetic to true), you will need your map and wizz wheel at the exam as you will do a flightplan, and just be careful when working out the wind corection angles and airspeeds as I only got one question wrong if I recall and it was because the flightplan had a 5kts lower airspeed and I misread it so this threw my calculations out.
Fuel consumption is another, also make sure you can read and understand nearly every symbol on the map.
Basically make sure you have a sound knowledge of Nav before taking the exam, I take it you have bought the PPL Confuser book? if not, you must must must get it if you struggle on the exams, and make sure you are getting 95% right inthe Confuser book too.

As for the other exams, well some are easy some are hard, i didnt really find any of them taxing but I messed up a tad on the Aircraft General because I didnt read the questions properly, so some of the best advice I can give is TAKE YOUR TIME, read and understand the question before answering them.

good luck

Dean.