The cost of some of the preparation courses can be very expensive, as you'll find out if you do a search on PPRuNe. I spoke to someone who did a prep course and they got some good hints and it taught them exactly what was needed to get through numerical, spacial, verbal and diagrammatic reasoning. However I got through the testing using a few books on numerical reasoning and data interpretation, and I hunted round and got some cubes questions from a couple of books also. Once you get used to the psych questions they are straightforward, and buying books will save you a lot of money compared with doing the course. Once you've done a few questions you'll know what your weak areas are, so spend time working on them. To find the right books, I looked at what Qantas have on the website and just looked around for books with similar questions. A trip to a couple of good bookshops should be all the searching you need to do.
And of course as someone said you need to have what they want as far as the behavioural questions go.
Hope this helps
QQ