Create 10 practical ways for tax optimization of secondary income that help private individuals in Germany legally reduce their tax burden and keep more of their income.

Context: [Secondary income, tax optimization, Germany, tax return, allowances, advertising costs, small business regulation, income tax, tax classes, tax deductibility]

Goal: [Use legal possibilities for tax savings on additional income, correct declaration of income, optimal use of allowances and flat rates, tips for the tax return, avoidance of typical mistakes and additional payments]

For each way explain:
1) a concrete measure or tactic for tax optimization of secondary income
2) practical advice for implementation and necessary requirements
3) opportunities, limits, and possible pitfalls in German tax law

The ways should:
- cover various types of secondary income (freelancing, sales, rental, online platforms, etc.)
- be understandable even for beginners without tax expertise
- include references to relevant forms and deadlines
- motivate conscious and responsible tax planning