I spent 13 years in the RAAF including 5 years instructing at 2FTS on the Macchi and PC9. My information may be old but hopefully, still relevant. As others have said, you will definately need to address your spelling and sentence construction when you are writing an essay otherwise you will draw unwarranted attention to yourself. Having said that, just about all the military (and civil) pilots I have ever met have been terrible at English and grammar so it's not that important, but you are trying to create the impression to a selection board that you have your act together. At the end of the day, they try and teach you how to write at Officer Training School and through other various avenues but if you have the raw skills, you will find it a lot easier.
At a selection board, they are trying to get a feel for how you will cope with the demands of flying training. It would help to be very familiar with the types of aircraft that are flown, where they are based, what squadrons operate them, a general understanding of the role of each aircraft and how they operate together and with other air forces. Having a knowledge of basic aircraft flight principles would be helpful. Try and find out what the role of a junior officer in the military is. When I went through it was just to drink heaps of piss and be a fool. When caught, you just had to say "guilty as charged sir" . Things have changed somewhat however and an officer is expected to not only perform his primary duty but also any number of secondary duties, some of which may be quite a task. In your annual Officer Evaluation Report, your performance in these secondary duties is reviewed. They are also trying to find out how motivated you are. The course is long and difficult and in 5 years instruction, I rarely met a student who didn't lose motivation at some stage. Motivation is essential. You can have the best aptitude score but if they don't think you are motivated to see it through, you won't get selected.
The flying training system is a lot better than what it used to be. The days of the abusive, angry instructor are long past. If you put in the effort, the system will do its best to pass you. To pass does require steady progression and you definately do have to have an ability to take criticism in a mature fashion and to go away and learn from the debrief. If you make it on to the course, you will generally have the raw ability - it's what you do with it that counts. I had a student once with low ability but the tenacity of ten men. He recently finished an F/A 18 exchange with the US Navy. Layman is correct when he says you are operating in an adult world. Integrity is paramount. If you can't be trusted in small things, how can you be trusted with that Hercules for a week overseas?
When it comes to dealing with psychologists and writing essays, be yourself. These people can spot bull**** a mile away.
Anyway, I hope to have helped. Best of luck with it all.