Mistake _ Not Tracking Your Spending and Expenses _ It is impossible to keep to your monthly budget if you fail to track your spending. It is recommended that you fill out both a weekly expenses report and a monthly expense record worksheet to track your spending and monthly expenses. It you don't track you spending you will not know what areas of your budget you need to adjust for next month.
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.