It takes guts to say no. No to coffee, no to drugs, no to "opportunities". Yes is easy. Most of us are natural peace keepers, so saying yes is the less turbulent response. But, if you stand for everything, you also stand for nothing.
Saying no requires conviction in your own ideas. It means that the easy thing might not always be the best thing. Sometimes we have to find our own path. Just because this is how things have always been done does not mean that's how they should continue to be done. Habits are hard to break.
The great innovators of the world, Einstein, Edison, Jobs, and even political revolutionaries like Gandhi, MLK, or Mandela all have one thing in common. They dare to question the status quo. Why? Why is this the way things are? What about electricity, relativity, civil rights?
It takes courage to say no, but that's how progress is made. Show conviction in your ideas. Dare to ask why. Practice saying no.