Although I went through selection as a cadet, I believe the selection proceedure was very similar for DEP and CEP applicants, but without the simulator day. Admittedly I went through a long time ago but I believe the tests are still pretty similar to this day. I did zero preparation as I was incredibly busy with other things at the time and BA was not the be all and end all. I was one of those scribbling down my logical workings in the maths questions, in the way I've always been trained to do - even if I already know the answer in my head. I completed significantly fewer maths questions than the rest of the applicants within my group, but I knew my answers were methodical and correct. I actually really enjoyed the whole selection experience, finding the computer tests challenging and really quite fun. I didn't over think anything. I just followed instructions carefully, read questions carefully, and answered interview questions as honestly as I could. I actually remember upon being asked "So why do you want be an airline pilot?", thinking "Crumbs man! This really is one question you could have prepared for!", but it resulted in my coming out with a completely honest answer.
The point I'm trying to make is that preparation is really just a means to get your mind thinking in the right ways and giving yourself the confidence to perform. I'd have thought these are all the kinds of things that a current pilot should already be able to do? I was a recent university science graduate, working in a very hand eye co-ordinated field at the time (although nothing to do with flying), so I was confident I could complete any tasks thrown at me. Perhaps my everyday skills were my preparation, but I had no idea what was coming on the testing days - only a knowledge that the test couldn't be as demanding as the kinds of things I could already do.