I have a slush fund for those knives that pop up that need an instant decision. (I established this habit from my past collecting of vintage guitars, I still have a fund waiting for a '69 Stratocaster)
I have the adjustable expenses that I can modify based on how much I have (ie food, saving etc), but then I have expenses that are always the same (ie rent, cell, gas, insurance, internet, TV etc).
After the necessities are taken care of, I divide whats left into how many days I have until I get paid again. I then decide on my adjustable expenses to determine how luxuriously to eat and how much goes towards saving. I have 2 saving categories, short term, and long term. Long term is saving with no goal, just for the sake of saving. Short term is to buy stuff that I want.
Most suggest deciding a fixed amount to save, but due to the nature of my work, my income varies so I don't have this ability. However I do assign percentages... as for saving, 60% towards long term, and 40% to wants and knives (mostly knives).