I stumbled upon this article recently : http://alexrecker.com/our-new-sid-meiers-civilization-inspired-budget/ and I thought it was a really neat way to follow your expenses.
Are you using a system of your own or do you follow a pre-existing thing ? What is it and what do you like/dislike about it ?
I started calcating my expenses last september and currently I use an Excel spreadsheet with fixed costs (school, rent, internet, bus/tube card, etc.) and variable costs (electricity/water/gaz, food, restaurants, events I'm going to) and other expenses (e.g. doctor, clothes, etc.)
It's easily updated : whenever I buy something I just add it to the excel sheet. But it doesn't really provide in-depth analysis of my expenses and how I could improve it.