Having just sat 2 CPL tests recently (failed the first, passed the second...phew), I think I managed to pick up a few of the old tricks. Managed to sail through the majority of the oral test of both with relative amount ease, except for one or two odd questions which had the opposite effect, undoing all my good work and immediately making me out to be a chump.
Agree totally with the previous posts: if you don't know the answer hold your hands up, say you're not sure and look it up in the flight manual or somewhere (and know where everything in there is too). As the saying goes: "better to keep your mouth shut and look a fool than to open it and remove all doubt". I had a few questions such as "what is the psi of the tyres?" and "if I was x feet over this town, would be be able to glide to x airfield?". Pedantic yes, but think it's just to show you know where you can look these things up if required.
As for the rest, the kind of things that initially threw me were the questions that were worded in such a way that the answer wasn't immediately obvious:
ie. You're flying in some moderate turbulence. A climb or descent is not available to you. What speed should you be flying at? Cue me reeling off every bull!!!!! answer in the book except the one he wanted. ("Below Va" was what he was after).
The rest of the questions really related to air law, particuarly met minima and thing like carriage dangerous goods, operations over water etc. Oh, and alternate fuel/oil grades usually comes up. I was always advised here to give the legal answer and the practical answer (if your oil was REALLY that low would you want to be putting a different grade in instead of calling someone out with the correct grade?)
best of luck