I just got back my results and got 44. I got a 6 in French which was a surprise because that had been the one I'd consistently got 7s in from the first year, so I suppose what I'm trying to demonstrate is that you shouldn't go into the IB with that mindset of 'I have to get 45 out of 45', because even if you do everything exactly right, you might miss a point or two due to nerves or due to a difficult paper.
The other thing is, try not to look at it as a total of 45 which goes down as you
lose points. It is better (and more appropriate) to look at it as a number starting from zero that's growing, as in any case, this course is meant to help you develop and grow in the two years, demonstrating a progress and growth in your 'number' as well.
There is only one method that is foolproof to getting 40+ (I don't want to say to getting 45, because luck can sway the top results). In my college and with everyone I know, the people who worked the hardest and studied the most, and truly committed to their coursework etc fully, were the ones who did better than they expected. Work out a really good revision plan and stick to it, and create a pattern in your life to your level of work per night. In my case, I'd got so used to sitting at my desk from 5 till 10 every night that it felt so weird when I finished to have got those hours back into my life, as I'd forgotten they existed outside revision. They now seem to drag aimlessly in comparison!
Finally, do not underestimate the power of coursework. It can make the difference between two grades. And coursework is not under exam conditions and you have plenty of time to complete it, so the first priority is to make that as perfect as you possibly can. It is the only area in which you can guarantee that your performance generally won't be affected by nerves or illness or a nasty surprise. So make the most of it!
Good luck, stay calm, and give yourself treats here and there, or you'll soon lose the will to live! And sitting at the other end now, I can say that the entire tedious process is worth it in the end.