I recently went through the BA selection and was successful. Before I went I went through this thread in detail, it's a brilliant resource, thanks to all those who have posted all this info to make it so.
Earlier on in the thread there's a bit of debate about paying for aptitude test prep and interview coaching for day one, and then paying for a 747 sim for day two. All I can say is that I didn't pay for any such preparation - now believe me, I definitely don't consider myself to be superman (and my assessment was far, far from perfect) yet I still managed to get through. It's my humble opinion that BA are looking for a 'fit' - if you are the sort of person that fits their mould then you will get through. Personally I would caution against spending too much money trying to practice beforehand.
As far as the sim goes, the information on this thread is spot on, it's clear that they're looking at CRM. Again, in my opinion the time spent in an expensive 747 sim 'practicing' would be better spent thinking carefully about how you might manage the workload of the limited number of types of scenario which you might conceivably get, especially in the context of working together with someone who is most probably from a different type/company.
Obviously all of the above is just my own opinion!
Best of luck to those who have yet to go through the process.