If you work it right, a prepaid debit card can actually help you monitor your budget and stay on track when it comes to what you spend each month. Of course there are all kinds of ways to get into financial trouble with any kind of debit or credit card, but with a prepaid card you eliminate a lot of the risk. All of your transactions are already paid for, and you cannot go over the amount that you have paid. Also, it is separate from your finances in your bank account if you have one, but you do not have to have one. These are a great option for someone who wants the convenience of debit but does not have a bank account.
To keep track of your budget using a prepaid debit card, start by deciding which expenses you will use it for. It may be that you want to use it for everything, but a wiser choice in the beginning it to decided that you will use your card for something specific. It may be groceries and gas, extra expenses, or any combination of whatever you may have to pay for monthly. Once you decide on which expenses you will pay with the card, set the budget for those expenses.
So if you budget $500 per month for gas and groceries and that is what you are going to use your card for, then put $500 on your prepaid debit card at the beginning of the month. This is all you have for those expenses for the month. Each time you buy either one, keep track of it. You can do this with a manual register like you would with a check book register at the time of purchase, or save the receipts and enter them in a spreadsheet at home. Find a system works for you and use it.
This helps a budget in a few ways. First, you are forced to keep up with the amount on the card so that you do not go hungry or run out of gas. Of course the situations may be different if you are using it for other types of expenses. But also, your other funds are free from those expenses and available for paying bills of whatever else you need them for. Once you get used to it you can add more expenses to the card, or have multiple prepaid debit cards for different expense categories to help you track your entire budget.