Spreadsheets are your friend - they're what I use whenever I run over budget. Make as detailed a spreadsheet as possible of your spending habits, referencing your bank statement, credit card statements, and if possible a log of what you've bought in the last week or so, and likewise of your income. Ways to save money will then present themselves.

From my experience, over-spend tends to happen because I'm not *aware* enough of what's going where, not for any other reason.