A budget in three numbers you can actually remember.
Split your net income into three buckets: 50% for needs like rent, groceries and insurance, 30% for wants like restaurants, travel and hobbies, 20% for saving and paying off debt. On €2,000 net that's €1,000, €600 and €400. If your fixed costs don't fit into the 50%, you know instantly where the problem is.
Useful: It doesn't replace a spreadsheet, but it shows you in a minute whether your cash flow is healthy.
Source: en.wikipedia.org