I know for many people, these system used is "as long as there's money in the account, keep spending!".
This is a nice article that gives everyone a nice system that can work. I'm not a fan of budgets or budgeting, but in order for this system to work, you do have to know at least what your major bills are each month. That way, you'll know how much to put into the "fixed" account each month.
Don't forget about timing of the bills and your paychecks. You don't have to have an entire month's worth of expenses in the fixed account if the timing of the deposits line up with when expenses have to be paid.
If you don't have a system, give this a try.
http://www.businessinsider.com/a-financial-planner-shares-her-personal-system-for-managing-money-2015-10
By the way, my wife and I did set a budget this past weekend, and accounted for all of our expenses, and accounted for every dollar of income and expenses. I fully recognize that it was a necessary exercise. It's going to take discipline but would help us achieve our financial goals.
