I basically added up all of my bills, annual expenses, potential plane tickets, and took them out of my yearly income. Then, I decided how much I should be able to save per month, and took that out. The remainder was divided by 12, and I came up with a monthly allowance.
This sounds lame, but, I keep a card with me that has the remaining allowance on it at all times. Whenever I spend, I subtract a rounded-to-the-dollar amount fron that allowance. This makes me think more about spending, realize what I’m spending on, and prioritize a little bit. I’ve been slipping recently, but I’m planning on subtracting a little from next month’s budget to make up for it.