I agree with this. Book them on payday and then you won't have the chance to spend the money. I was also terrible with money (still am sometimes) but you need to prioritise. It's only for a little while until you've passed etc. Then once you've got the job you want, you can spend it again on what you like. Although by then it might of helped you a bit with pointless spending.
As other people have said, curb the smoking too! That takes a lot of your money.