Defender Teamwork
As a defender you're never playing alone: every card you lead is also a signal to your partner, and every card you save is a promise about what you're holding. Read the position right, and two hands work like one.
You lead the Nine of Hearts, a suit your partner is out of. The Club Jack comes down and the trick is theirs.
Lead the active suit
When nothing sharper offers itself, lead the suit with the most cards still outstanding. It's the suit most likely to still be live for everyone at the table, so it keeps the pressure on declarer instead of feeding a suit that's already run dry.
Once trump is gone
The moment the declarer has no trump left, every ruff threat disappears with it. Lead your own top-of-suit winners freely from then on: nothing can steal them from you anymore.
Setting up partner's trump
Your partner is void in a suit the declarer still holds. Lead exactly that suit and your partner trumps in for the trick, spending a low trump for a win neither of you could get any other way.
Returning partner's suit
Partner led a suit and won the trick with it. That's proof the suit is strong in their hand, so send the lowest card you have straight back into it and let them keep working it.
Long to the enemy
"Dem Freunde kurz, dem Feinde lang." When the declarer plays right after you, lead your longest suit at them: a long suit is harder to escape from, and it's the declarer who has to keep answering it.
Short to a friend
"Dem Freunde kurz, dem Feinde lang." When your partner plays right after you instead, lead your shortest suit: you empty your hand of it fast, which sets you up to trump in later when it comes back around.