ecco123
Well, it goes without saying that you need to be brilliant to get a place on the programme. However, if you know what it takes to be a pilot, and you put in a lot of effort to prepare in the right way, you can't go wrong. I know some guys on the course at Jerez and what they felt made them successful at selection was the preparation they put into it. They knew what was expected of a BA pilot, and they prepared accordingly. You can prepare for just about every aspect of selection, apart from the final BA aptitude test.
I would definitely recommend getting yourself some practice aptitude testing software, however I would wait until you know that you've been invited to attend stage 2. Getting to stage 2 is not easy in itself, and prep software is not cheap, so you should just focus on the application form, CV and cover letter for the time being.