We just upgraded the Budgets/Spending Limits tool by adding a "Rollover" option. This option, if enabled, will let your spending limits rollover each month. This means that any unused money will be added to the limit for the following month.
Here's an example: You have a limit set on Entertainment for $100 a month. Lets say you only spend $50 this month. When the limit resets, your new limit will be $150 for the next month.
The opposite is also true though. If you spend more than your limit, your limit for the next month will be smaller by the amount you over spent.
You'll be able to tell which limits are set to rollover by an icon next to the limit amount.