You have to set some financial long term goals, there must be something you really want to buy/do that will require quite a sum of money, this should give you enough motivation to stop you from making impulsive or unnecessary purchases and eventually reaching said financial goal. I myself have been saving for quite some time for overseas travel and it helps reassure myself that small time purchases does not compare to seeing the world.
You really just gotta set a goal for yourself and try your hardest to achieve it. I'm striving towards $600 for a final gamertag right now, but, due to financial problems, that might not happen for a while.