I'm a fan of MS Excel _ my apologies for those who have issues with Microsoft _ because I find its powerful calculations are easy to manipulate to help me better understand my finances. It doesn't really matter though which spreadsheeting package you use so long as you are able to calculate your amounts effectively. There are commercial budgeting worksheets available but if you want to create your own its a simple process.
Mistake _ Not Planning for Upcoming Expenses _ When you are creating a monthly budget it is always helpful to save a certain amount of money for a given category even if you don't have that expense for the given month. For example, let's say you pay your car insurance every six month. It is important to take your payment and divide it by six. Then, each month you will want to save that amount so you have the insurance payment when it comes due instead of trying to come up with the whole amount in one given month. If you can do this for as many periodic expenses as possible, it will be easier to have a balanced budget each month.