Determine Your Actual Needs
It's easy to forget about your budget when you see something you really like on sale or if you feel an impulse to buy something you normally would. To avoid making decisions that will ruin your budget, you need to really be honest with yourself and decide what you actually need and what you only want. If you recognize where you draw your needs/wants line, you'll be able to better resist things that would wreck your budget.

Use the Envelope System
Many people create trouble for themselves by using plastic too often - it's much easier to pay for something using a credit card because it almost feels like you're not paying for it. Paying with cash is the opposite - using cash feels more concrete and when it's gone, it's gone. You can use this as a tool to stick to your budget. Set an amount for each area of your budget - groceries, gas, clothing, entertainment, etc. Withdraw the amount for each area in cash from your bank and put the amount in each corresponding envelope. When you go shopping, only take the envelope you need and no other form of payment - once the money is gone, you're done. Willpower doesn't always work and this strategy takes that element out, helping you to follow your budget better.

Create a Cushion
You may be trying your very best to live on your set budget, but then some unexpected expense comes up and throws your whole plan off. Pretty soon, you're back to where you started, unstructured and spending more than you make. However, if you have some money in savings designated for unexpected expenses (like car repairs, illness, home maintenance, etc.), you can fall back on that without upsetting your budget and the progress you've made.

Have Some Fun Money
Living on a budget isn't easy - it takes discipline and hard work. Often, when someone is pinching pennies, they can start to feel poor and deprived. These feelings are not only depressing, but they spell trouble for your budgeting goals. When you feel deprived, you may find yourself in a moment of weakness: you're at the mall, you see something you usually bought in your more extravagant past, and you decide you are tired of going without. So you splurge - only to get home and realize you've blown the rest of your paycheck on shoes, leaving you with even more negative feelings. You can avoid these feelings of deprivation by allowing yourself (and anyone else following the budget) a little bit of fun spending money that you can blow on anything you want. Not only will you not feel deprived, but you will also be happier and more likely to stick to your budget.