To get successful, you had to say yes to everything.
But once you are successful, you have to change your strategy. If you continue doing that, you'll drown in all the opportunities.
Now say NO to anything that makes you say anything less than "Hell yeah!"

But I am still not wholly agree with that. Saying NO is better even if you are new, not yet successful. Success is successfully executing something you started. So if you say YES to ten things and succeed in 3 things, are you successful? NO, you are not. So saying YES failed to make you successful. 

Now say if you said NO to 7 things and successfully completed 3 things. Are you successful by saying NO? Yes, your success rate is 100%. 

Focus on saying NO to everything that you think your success rate is poor. I don't mean not taking a risk and playing safe. I mean, can you focus on many things at once and succeed in them?

The success rate will get you better opportunities and connections.