What Type of Dummy?

As a defender, you should try to put yourself in declarer's shoes. What are they going to use the dummy for, exactly? 

Sometimes, the answer is clear: a long suit or shortness is sticking out at you. Other times, you dones' necessarily see anything particularly useful. It's important to take this step to put you in the right direction. 

Shortness

If you see shortness in dummy, think about leading trumps. There will often be a hint to do this (if declarer doesn't lead trump, you should). 

Long Suit

Think about how you can cut declarer off from a long suit by attacking entries whenever they have a long suit. Sometimes, you might even play that suit to ensure that declarer is stuck in dummy at the wrong time. 

Nothing Interesting

When you don't see anything dangerous, think "passive". Avoid helping declarer by breaking new suits. Instead, try to find the way to get out of your hand and make declarer work for everything.