If after paying or allocating expenses from your average income, you find that you have a surplus, it may not mean that you have extra funds to spend freely.
Before you plan a monthly shopping spree or pub crawl, ensure that you have properly coordinated your expenses with your average income. Here are a few places to double-check:

• Income: ensure that you have calculated an average income based from a few months or pay periods. If you are only using your last month's pay stubs, they may not accurately reflect what you really earn.
• Expenses: Are you deducting or allocating enough for each of your expense categories? Your may have a surplus because you haven't set aside enough funds for utilities, home and cellular phone expenses. Although you are trying to minimize, failing to allocate or deduct a large enough amount to realistically cover your fixed expenses may throw your budget off balance.
• Reserves: Since budgets are never perfect, and expenses may fluctuate, ensure that you have set aside or allocated a reserve fund in case of temporary overspending, which is always possible (even on fixed expenses).

If after checking your average income, proposed expense payment or allocations, and reserves, you still have a surplus, make sure you allocate or deduct all of the funds into some expense category. If you have a "fun" category, it looks as if you can have more fun each budgeting period than you thought. If you have a savings category, you can put even more away for retirement.

Make Sure You're Saving

With whatever type of budget you decide to use, whether income-and-expense or zero-based, make sure you put aside some income for savings. You may already have particular savings goals outlined (see "Savings Goals"), which may be motivating you to create a budget right now. Or maybe you've never been concerned about putting anything away (as most Americans are not), but are curious as to how much you're really spending on daily lattes and upscale dinners, and want to achieve some control over your finances.

Either way, it is likely that after calculating your average income and realistic expenses, you have more money available to you than you thought before. It is wise to put these funds aside, thereby "paying yourself first". By doing so, you have the psychological reward of knowing that you adhered to your budget, and were smart about how you spent your money.

With that in mind, make sure you allocate or categorize these surplus funds in some way. Whether you decide to save it for the long-term, or spend it once all of your expenses have been paid is up to you.

Keep in mind that not saving money in some capacity increases could increase your expenditure during emergencies and other major events, as you may be using financing to pay for momentary costs that could have otherwise been paid in cash. Among other factors, failing to save is an indicator of poor financial health.

If you need help starting long- or short-term savings plan, please see the budgeting-help.com articles on "Saving for (Any) Day".

References
Mgichome.com. Develop a Budget
Howtodothings.com. How to Save Money
Gettingfinancesdone.com. 3 Reasons Most Budgets don't Work
Gettingfinancesdone.com. What is a Zero Based Budget
Erin Huffstetler. Debt Free Living