"How do I say 'no' to extra work without hurting my career?"

The key is saying 'not this' instead of 'not anything.' Show you're committed, but realistic. Try: 'I want to deliver quality on my current projects. If I take this on, something will suffer. Can you help me prioritize?' This puts the ball in their court to decide what matters most. Or: 'I can take this on, but it would push [other project] back by a week. Is that okay, or should we find another solution?' Never apologize excessively or explain your personal life. 'I have some commitments that week' is enough. You don't owe details. And pick your battles. Saying yes sometimes, especially for visible/important work, builds capital for the times you need to say no. If you say no to everything, people stop asking—and stop thinking of you for opportunities too.